{"id":714,"date":"2016-11-04T03:34:58","date_gmt":"2016-11-04T03:34:58","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/microbiology\/?post_type=chapter&#038;p=714"},"modified":"2018-07-11T19:27:22","modified_gmt":"2018-07-11T19:27:22","slug":"the-language-of-epidemiologists","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/chapter\/the-language-of-epidemiologists\/","title":{"raw":"The Language of Epidemiologists","rendered":"The Language of Epidemiologists"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\n<ul>\r\n \t<li>Explain the difference between prevalence and incidence of disease<\/li>\r\n \t<li>Distinguish the characteristics of sporadic, endemic, epidemic, and pandemic diseases<\/li>\r\n \t<li>Explain the use of Koch\u2019s postulates and their modifications to determine the etiology of disease<\/li>\r\n \t<li>Explain the relationship between epidemiology and public health<\/li>\r\n<\/ul>\r\n<\/div>\r\nThe field of <strong>epidemiology<\/strong> concerns the geographical distribution and timing of infectious disease occurrences and how they are transmitted and maintained in nature, with the goal of recognizing and controlling outbreaks. The science of epidemiology includes <strong>etiology<\/strong> (the study of the causes of disease) and investigation of disease transmission (mechanisms by which a disease is spread).\r\n<h2>Analyzing Disease in a Population<\/h2>\r\nEpidemiological analyses are always carried out with reference to a population, which is the group of individuals that are at risk for the disease or condition. The population can be defined geographically, but if only a portion of the individuals in that area are susceptible, additional criteria may be required. Susceptible individuals may be defined by particular behaviors, such as intravenous drug use, owning particular pets, or membership in an institution, such as a college. Being able to define the population is important because most measures of interest in epidemiology are made with reference to the size of the population.\r\n\r\nThe state of being diseased is called <strong>morbidity<\/strong>. Morbidity in a population can be expressed in a few different ways. Morbidity or total morbidity is expressed in numbers of individuals without reference to the size of the population. The <strong>morbidity rate<\/strong> can be expressed as the number of diseased individuals out of a standard number of individuals in the population, such as 100,000, or as a percent of the population.\r\n\r\n[caption id=\"\" align=\"alignright\" width=\"450\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03165426\/OSC_Microbio_16_01_IncPrev.jpg\" alt=\"Graph of HIV prevalence and incidence in the US 1980-2010. The number o people living with HIV\/AIDS was near 0 in 1980 and has increased steadily to over 1 million. There as a short plateau from 1990 to 1995. The number of new infections increased to nearly 200,000 in 1985 and dropped until 1990. It remains steady at somewhere near 50,000.\" width=\"450\" height=\"273\" \/> Figure\u00a01. This graph compares the incidence of HIV (the number of new cases reported each year) with the prevalence (the total number of cases each year). Prevalence and incidence can also be expressed as a rate or proportion for a given population.[\/caption]\r\n\r\nThere are two aspects of morbidity that are relevant to an epidemiologist: a disease\u2019s <strong>prevalence<\/strong> and its <strong>incidence<\/strong>. Prevalence is the number, or proportion, of individuals with a particular illness in a given population at a point in time. For example, the <strong>Centers for Disease Control and Prevention<\/strong> (CDC) estimated that in 2012, there were about 1.2 million people 13 years and older with an active human immunodeficiency virus (<strong>HIV<\/strong>) infection. Expressed as a proportion, or rate, this is a prevalence of 467 infected persons per 100,000 in the population.[footnote]H. Irene Hall, Qian An, Tian Tang, Ruiguang Song, Mi Chen, Timothy Green, and Jian Kang. \"Prevalence of Diagnosed and Undiagnosed HIV Infection\u2014United States, 2008\u20132012.\" <em>Morbidity and Mortality Weekly Report<\/em> 64, no. 24 (2015): 657\u2013662.[\/footnote] On the other hand, incidence is the number or proportion of <em>new<\/em> cases in a period of time. For the same year and population, the CDC estimates that there were 43,165 newly diagnosed cases of HIV infection, which is an incidence of 13.7 new cases per 100,000 in the population.[footnote]Centers for Disease Control and Prevention. \"Diagnoses of HIV Infection in the United States and Dependent Areas, 2014.\" <em>HIV Surveillance Report<\/em> 26 (2015).[\/footnote] The relationship between incidence and prevalence can be seen in Figure\u00a01. For a chronic disease like HIV infection, prevalence will generally be higher than incidence because it represents the cumulative number of new cases over many years minus the number of cases that are no longer active (e.g., because the patient died or was cured).\r\n\r\nIn addition to morbidity rates, the incidence and prevalence of <strong>mortality<\/strong> (death) may also be reported. A <strong>mortality rate<\/strong> can be expressed as the percentage of the population that has died from a disease or as the number of deaths per 100,000 persons (or other suitable standard number).\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Think about It<\/h3>\r\n<ul>\r\n \t<li>Explain the difference between incidence and prevalence.<\/li>\r\n \t<li>Describe how morbidity and mortality rates are expressed.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<h2>Patterns of Incidence<\/h2>\r\nDiseases that are seen only occasionally, and usually without geographic concentration, are called <strong>sporadic disease<\/strong><strong>s<\/strong>. Examples of sporadic diseases include <strong>tetanus<\/strong>, <strong>rabies<\/strong>, and <strong>plague<\/strong>. In the United States, <strong><em>Clostridium tetani<\/em><\/strong>, the bacterium that causes tetanus, is ubiquitous in the soil environment, but incidences of infection occur only rarely and in scattered locations because most individuals are vaccinated, clean wounds appropriately, or are only rarely in a situation that would cause infection.[footnote]Centers for Disease Control and Prevention. \"Tetanus Surveillance\u2014United States, 2001\u20132008.\" <em>Morbidity and Mortality Weekly Report<\/em> 60, no. 12 (2011): 365.[\/footnote] Likewise in the United States there are a few scattered cases of plague each year, usually contracted from rodents in rural areas in the western states.[footnote]Centers for Disease Control and Prevention. \"Plague in the United States.\" 2015. http:\/\/www.cdc.gov\/plague\/maps. Accessed June 1, 2016.[\/footnote]\r\n\r\nDiseases that are constantly present (often at a low level) in a population within a particular geographic region are called <strong>endemic disease<\/strong><strong>s<\/strong>. For example, <strong>malaria<\/strong> is endemic to some regions of Brazil, but is not endemic to the United States.\r\n\r\nDiseases for which a larger than expected number of cases occurs in a short time within a geographic region are called <strong>epidemic disease<\/strong><strong>s<\/strong>. Influenza is a good example of a commonly epidemic disease. Incidence patterns of influenza tend to rise each winter in the northern hemisphere. These seasonal increases are expected, so it would not be accurate to say that influenza is epidemic every winter; however, some winters have an usually large number of seasonal influenza cases in particular regions, and such situations would qualify as epidemics (Figure\u00a02\u00a0and Figure\u00a03).\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"701\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03165430\/OSC_Microbio_16_01_Flu.jpg\" alt=\"A graph of the percentage of emergency department visits for influenza-like illness. The X axis is times of the year and the Y axis is percent. The national baseline is near 2.5%. All yeas have a small peak in January and a larger peak February to April. 2007-2008 had the largest peak in February to April. 2008 \u2013 2009 had an additional peak from May to September.\" width=\"701\" height=\"448\" \/> Figure\u00a02. The 2007\u20132008 influenza season in the United States saw much higher than normal numbers of visits to emergency departments for influenza-like symptoms as compared to the previous and the following years. (credit: modification of work by Centers for Disease Control and Prevention)[\/caption]\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"699\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03165433\/OSC_Microbio_16_01_Flu2.jpg\" alt=\"A graph of the percentage of all deaths due to influenza an pneumonia. The X axis is years and the Y axis is percent. The seasonal baseline fluctuates to a high in winter. The epidemic threshold is just a half percent higher than the baseline. The actual mortality fluctuates above and below both of these lines.\" width=\"699\" height=\"408\" \/> Figure\u00a03. The seasonal epidemic threshold (blue curve) is set by the CDC-based data from the previous five years. When actual mortality rates exceed this threshold, a disease is considered to be epidemic. As this graph shows, pneumonia- and influenza-related mortality saw pronounced epidemics during the winters of 2003\u20132004, 2005, and 2008. (credit: modification of work by Centers for Disease Control and Prevention)[\/caption]\r\n\r\nAn epidemic disease signals the breakdown of an equilibrium in disease frequency, often resulting from some change in environmental conditions or in the population. In the case of influenza, the disruption can be due to antigenic shift or drift (see <a href=\".\/chapter\/virulence-factors-of-bacterial-and-viral-pathogens\/\" target=\"_blank\" rel=\"noopener\">Virulence Factors of Bacterial and Viral Pathogens<\/a>), which allows influenza virus strains to circumvent the acquired immunity of their human hosts.\r\n\r\nAn epidemic that occurs on a worldwide scale is called a <strong>pandemic disease<\/strong>. For example, <strong>HIV\/AIDS<\/strong> is a pandemic disease and novel influenza virus strains often become pandemic.\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Think about It<\/h3>\r\n<ul>\r\n \t<li>Explain the difference between sporadic and endemic disease.<\/li>\r\n \t<li>Explain the difference between endemic and epidemic disease.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<h2>Etiology<\/h2>\r\nWhen studying an epidemic, an epidemiologist\u2019s first task is to determinate the cause of the disease, called the <strong>etiologic agent<\/strong> or <strong>causative agent<\/strong>. Connecting a disease to a specific pathogen can be challenging because of the extra effort typically required to demonstrate direct causation as opposed to a simple association. It is not enough to observe an association between a disease and a suspected pathogen; controlled experiments are needed to eliminate other possible causes. In addition, pathogens are typically difficult to detect when there is no immediate clue as to what is causing the outbreak. Signs and symptoms of disease are also commonly nonspecific, meaning that many different agents can give rise to the same set of signs and symptoms. This complicates diagnosis even when a causative agent is familiar to scientists.\r\n\r\nRobert <strong>Koch<\/strong> was the first scientist to specifically demonstrate the causative agent of a disease (anthrax) in the late 1800s. Koch developed four criteria, now known as <strong>Koch\u2019s postulates<\/strong>, which had to be met in order to positively link a disease with a pathogenic microbe. Without Koch\u2019s postulates, the <strong>Golden Age of Microbiology<\/strong> would not have occurred. Between 1876 and 1905, many common diseases were linked with their etiologic agents, including cholera, diphtheria, gonorrhea, meningitis, plague, syphilis, tetanus, and tuberculosis. Today, we use the <strong>molecular Koch\u2019s postulates<\/strong>, a variation of Koch\u2019s original postulates that can be used to establish a link between the disease state and virulence traits unique to a pathogenic strain of a microbe. Koch\u2019s original postulates and molecular Koch\u2019s postulates were described in more detail in <a href=\".\/chapter\/how-pathogens-cause-disease\/\" target=\"_blank\" rel=\"noopener\">How Pathogens Cause Disease<\/a>.\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Think about It<\/h3>\r\n<ul>\r\n \t<li>List some challenges to determining the causative agent of a disease outbreak.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<h2>The Role of Public Health Organizations<\/h2>\r\nThe main national public health agency in the United States is the <strong>Centers for Disease Control and Prevention (CDC)<\/strong>, an agency of the Department of Health and Human Services. The CDC is charged with protecting the public from disease and injury. One way that the CDC carries out this mission is by overseeing the <strong>National Notifiable Disease Surveillance System<\/strong> (NNDSS) in cooperation with regional, state, and territorial public health departments. The NNDSS monitors diseases considered to be of public health importance on a national scale. Such diseases are called <strong>notifiable disease<\/strong><strong>s<\/strong> or <strong>reportable disease<\/strong><strong>s<\/strong> because all cases must be reported to the CDC. A physician treating a patient with a notifiable disease is legally required to submit a report on the case. Notifiable diseases include <strong>HIV<\/strong> infection, <strong>measles<\/strong>, <strong>West Nile virus<\/strong> infections, and many others. Some states have their own lists of notifiable diseases that include diseases beyond those on the CDC\u2019s list.\r\n\r\nNotifiable diseases are tracked by epidemiological studies and the data is used to inform health-care providers and the public about possible risks. The CDC publishes the <strong><em>Morbidity and Mortality Weekly Report<\/em> (<em>MMWR<\/em>)<\/strong>, which provides physicians and health-care workers with updates on public health issues and the latest data pertaining to notifiable diseases. Table 1\u00a0is an example of the kind of data contained in the <em>MMWR<\/em>.\r\n<table id=\"fs-id1167660338485\" class=\"span-all\" summary=\"Table titled: Incidence of Four Notifiable Diseases in the United States, Week Ending January 2, 2016. Columns: Disease; Current Week (Jan 2, 2016); Median of Previous 52 weeks; Maximum of previous 52 weeks; Cumulative Cases 2015. Campylobacteriosis: 406; 869; 1,385; 46,618. Chlamydia trachomatis infection: 11,024; l28,562; 31,089; 1,425,303. Giardiasis: 115; 230; 335; 11,870. Gonorrhea: 3,207; 7,155; 8,283; 369,926.\">\r\n<thead>\r\n<tr>\r\n<th colspan=\"5\">Table 1. Incidence of Four Notifiable Diseases in the United States, Week Ending January 2, 2016<\/th>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<th>Disease<\/th>\r\n<th>Current Week (Jan 2, 2016)<\/th>\r\n<th>Median of Previous 52 Weeks<\/th>\r\n<th>Maximum of Previous 52 Weeks<\/th>\r\n<th>Cumulative Cases 2015<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr valign=\"top\">\r\n<td>Campylobacteriosis<\/td>\r\n<td>406<\/td>\r\n<td>869<\/td>\r\n<td>1,385<\/td>\r\n<td>46,618<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td><em>Chlamydia trachomatis<\/em> infection<\/td>\r\n<td>11,024<\/td>\r\n<td>28,562<\/td>\r\n<td>31,089<\/td>\r\n<td>1,425,303<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Giardiasis<\/td>\r\n<td>115<\/td>\r\n<td>230<\/td>\r\n<td>335<\/td>\r\n<td>11,870<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Gonorrhea<\/td>\r\n<td>3,207<\/td>\r\n<td>7,155<\/td>\r\n<td>8,283<\/td>\r\n<td>369,926<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<div class=\"textbox\">The current <a href=\"http:\/\/www.cdc.gov\/mmwr\/index.html\"><em>Morbidity and Mortality Weekly Report<\/em><\/a> is available online.<\/div>\r\n<div class=\"textbox key-takeaways\">\r\n<h3>\u00a0Think about It<\/h3>\r\n<ul>\r\n \t<li>Describe how health agencies obtain data about the incidence of diseases of public health importance.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Key Concepts and Summary<\/h3>\r\n<ul>\r\n \t<li><strong>Epidemiology<\/strong> is the science underlying public health.<\/li>\r\n \t<li><strong>Morbidity<\/strong> means being in a state of illness, whereas <strong>mortality<\/strong> refers to death; both <strong>morbidity rates<\/strong> and <strong>mortality rates<\/strong> are of interest to epidemiologists.<\/li>\r\n \t<li><strong>Incidence<\/strong> is the number of new cases (morbidity or mortality), usually expressed as a proportion, during a specified time period; <strong>prevalence<\/strong> is the total number affected in the population, again usually expressed as a proportion.<\/li>\r\n \t<li><strong>Sporadic diseases<\/strong> only occur rarely and largely without a geographic focus. <strong>Endemic diseases<\/strong> occur at a constant (and often low) level within a population. <strong>Epidemic diseases<\/strong> and <strong>pandemic diseases<\/strong> occur when an outbreak occurs on a significantly larger than expected level, either locally or globally, respectively.<\/li>\r\n \t<li><strong>Koch\u2019s postulates<\/strong> specify the procedure for confirming a particular pathogen as the etiologic agent of a particular disease. Koch\u2019s postulates have limitations in application if the microbe cannot be isolated and cultured or if there is no animal host for the microbe. In this case, molecular Koch\u2019s postulates would be utilized.<\/li>\r\n \t<li>In the United States, the <strong>Centers for Disease Control and Prevention<\/strong> monitors <strong>notifiable diseases<\/strong> and publishes weekly updates in the <em>Morbidity and Mortality Weekly Report.<\/em><\/li>\r\n<\/ul>\r\n<\/div>\r\n<div class=\"textbox exercises\">\r\n<h3>Matching<\/h3>\r\nMatch each term with its description.\r\n<table id=\"fs-id1167662472243\" class=\"unnumbered unstyled\" summary=\"sporadic disease; endemic disease; pandemic disease; morbidity rate; mortality rate. A. the number of disease cases per 100,000 individuals. B. a disease in higher than expected numbers around the world. C. the number of deaths from a disease for every 10,000 individuals. D. a disease found occasionally in a region with cases occurring mainly in isolation from each other. E. a disease found regularly in a region.\">\r\n<tbody>\r\n<tr valign=\"top\">\r\n<td>___sporadic\u00a0disease<\/td>\r\n<td>A. the number of disease cases per 100,000 individuals<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>___endemic\u00a0disease<\/td>\r\n<td>B. a disease in higher than expected numbers around the world<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>___pandemic\u00a0disease<\/td>\r\n<td>C. the number of deaths from a disease for every 10,000 individuals<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>___morbidity\u00a0rate<\/td>\r\n<td>D. a disease found occasionally in a region with cases occurring mainly in isolation from each other<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>___mortality\u00a0rate<\/td>\r\n<td>E. a disease found regularly in a region<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n[reveal-answer q=\"524314\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"524314\"]\r\n<ol>\r\n \t<li>(D) Sporadic disease is\u00a0a disease found occasionally in a region with cases occurring mainly in isolation from each other.<\/li>\r\n \t<li>(E) Endemic disease is\u00a0a disease found regularly in a region.<\/li>\r\n \t<li>(B) Pandemic disease is\u00a0a disease in higher than expected numbers around the world.<\/li>\r\n \t<li>(A) Morbidity rate is the number of disease cases per 100,000 individuals.<\/li>\r\n \t<li>(C) Mortality rate is\u00a0the number of deaths from a disease for every 10,000 individuals.<\/li>\r\n<\/ol>\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox exercises\">\r\n<h3>Fill in the Blank<\/h3>\r\nThe ________ collects data and conducts epidemiologic studies in the United States.\r\n[reveal-answer q=\"261307\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"261307\"]The <strong>CDC (Centers for Disease Control and Prevention)<\/strong> collects data and conducts epidemiologic studies in the United States.[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Think about It<\/h3>\r\n<ol>\r\n \t<li>During an epidemic, why might the prevalence of a disease at a particular time not be equal to the sum of the incidences of the disease?<\/li>\r\n \t<li>In what publication would you find data on emerging\/reemerging diseases in the United States?<\/li>\r\n \t<li>Why might an epidemiological population in a state not be the same size as the number of people in a state? Use an example.<\/li>\r\n<\/ol>\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<ul>\n<li>Explain the difference between prevalence and incidence of disease<\/li>\n<li>Distinguish the characteristics of sporadic, endemic, epidemic, and pandemic diseases<\/li>\n<li>Explain the use of Koch\u2019s postulates and their modifications to determine the etiology of disease<\/li>\n<li>Explain the relationship between epidemiology and public health<\/li>\n<\/ul>\n<\/div>\n<p>The field of <strong>epidemiology<\/strong> concerns the geographical distribution and timing of infectious disease occurrences and how they are transmitted and maintained in nature, with the goal of recognizing and controlling outbreaks. The science of epidemiology includes <strong>etiology<\/strong> (the study of the causes of disease) and investigation of disease transmission (mechanisms by which a disease is spread).<\/p>\n<h2>Analyzing Disease in a Population<\/h2>\n<p>Epidemiological analyses are always carried out with reference to a population, which is the group of individuals that are at risk for the disease or condition. The population can be defined geographically, but if only a portion of the individuals in that area are susceptible, additional criteria may be required. Susceptible individuals may be defined by particular behaviors, such as intravenous drug use, owning particular pets, or membership in an institution, such as a college. Being able to define the population is important because most measures of interest in epidemiology are made with reference to the size of the population.<\/p>\n<p>The state of being diseased is called <strong>morbidity<\/strong>. Morbidity in a population can be expressed in a few different ways. Morbidity or total morbidity is expressed in numbers of individuals without reference to the size of the population. The <strong>morbidity rate<\/strong> can be expressed as the number of diseased individuals out of a standard number of individuals in the population, such as 100,000, or as a percent of the population.<\/p>\n<div style=\"width: 460px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03165426\/OSC_Microbio_16_01_IncPrev.jpg\" alt=\"Graph of HIV prevalence and incidence in the US 1980-2010. The number o people living with HIV\/AIDS was near 0 in 1980 and has increased steadily to over 1 million. There as a short plateau from 1990 to 1995. The number of new infections increased to nearly 200,000 in 1985 and dropped until 1990. It remains steady at somewhere near 50,000.\" width=\"450\" height=\"273\" \/><\/p>\n<p class=\"wp-caption-text\">Figure\u00a01. This graph compares the incidence of HIV (the number of new cases reported each year) with the prevalence (the total number of cases each year). Prevalence and incidence can also be expressed as a rate or proportion for a given population.<\/p>\n<\/div>\n<p>There are two aspects of morbidity that are relevant to an epidemiologist: a disease\u2019s <strong>prevalence<\/strong> and its <strong>incidence<\/strong>. Prevalence is the number, or proportion, of individuals with a particular illness in a given population at a point in time. For example, the <strong>Centers for Disease Control and Prevention<\/strong> (CDC) estimated that in 2012, there were about 1.2 million people 13 years and older with an active human immunodeficiency virus (<strong>HIV<\/strong>) infection. Expressed as a proportion, or rate, this is a prevalence of 467 infected persons per 100,000 in the population.<a class=\"footnote\" title=\"H. Irene Hall, Qian An, Tian Tang, Ruiguang Song, Mi Chen, Timothy Green, and Jian Kang. &quot;Prevalence of Diagnosed and Undiagnosed HIV Infection\u2014United States, 2008\u20132012.&quot; Morbidity and Mortality Weekly Report 64, no. 24 (2015): 657\u2013662.\" id=\"return-footnote-714-1\" href=\"#footnote-714-1\" aria-label=\"Footnote 1\"><sup class=\"footnote\">[1]<\/sup><\/a> On the other hand, incidence is the number or proportion of <em>new<\/em> cases in a period of time. For the same year and population, the CDC estimates that there were 43,165 newly diagnosed cases of HIV infection, which is an incidence of 13.7 new cases per 100,000 in the population.<a class=\"footnote\" title=\"Centers for Disease Control and Prevention. &quot;Diagnoses of HIV Infection in the United States and Dependent Areas, 2014.&quot; HIV Surveillance Report 26 (2015).\" id=\"return-footnote-714-2\" href=\"#footnote-714-2\" aria-label=\"Footnote 2\"><sup class=\"footnote\">[2]<\/sup><\/a> The relationship between incidence and prevalence can be seen in Figure\u00a01. For a chronic disease like HIV infection, prevalence will generally be higher than incidence because it represents the cumulative number of new cases over many years minus the number of cases that are no longer active (e.g., because the patient died or was cured).<\/p>\n<p>In addition to morbidity rates, the incidence and prevalence of <strong>mortality<\/strong> (death) may also be reported. A <strong>mortality rate<\/strong> can be expressed as the percentage of the population that has died from a disease or as the number of deaths per 100,000 persons (or other suitable standard number).<\/p>\n<div class=\"textbox key-takeaways\">\n<h3>Think about It<\/h3>\n<ul>\n<li>Explain the difference between incidence and prevalence.<\/li>\n<li>Describe how morbidity and mortality rates are expressed.<\/li>\n<\/ul>\n<\/div>\n<h2>Patterns of Incidence<\/h2>\n<p>Diseases that are seen only occasionally, and usually without geographic concentration, are called <strong>sporadic disease<\/strong><strong>s<\/strong>. Examples of sporadic diseases include <strong>tetanus<\/strong>, <strong>rabies<\/strong>, and <strong>plague<\/strong>. In the United States, <strong><em>Clostridium tetani<\/em><\/strong>, the bacterium that causes tetanus, is ubiquitous in the soil environment, but incidences of infection occur only rarely and in scattered locations because most individuals are vaccinated, clean wounds appropriately, or are only rarely in a situation that would cause infection.<a class=\"footnote\" title=\"Centers for Disease Control and Prevention. &quot;Tetanus Surveillance\u2014United States, 2001\u20132008.&quot; Morbidity and Mortality Weekly Report 60, no. 12 (2011): 365.\" id=\"return-footnote-714-3\" href=\"#footnote-714-3\" aria-label=\"Footnote 3\"><sup class=\"footnote\">[3]<\/sup><\/a> Likewise in the United States there are a few scattered cases of plague each year, usually contracted from rodents in rural areas in the western states.<a class=\"footnote\" title=\"Centers for Disease Control and Prevention. &quot;Plague in the United States.&quot; 2015. http:\/\/www.cdc.gov\/plague\/maps. Accessed June 1, 2016.\" id=\"return-footnote-714-4\" href=\"#footnote-714-4\" aria-label=\"Footnote 4\"><sup class=\"footnote\">[4]<\/sup><\/a><\/p>\n<p>Diseases that are constantly present (often at a low level) in a population within a particular geographic region are called <strong>endemic disease<\/strong><strong>s<\/strong>. For example, <strong>malaria<\/strong> is endemic to some regions of Brazil, but is not endemic to the United States.<\/p>\n<p>Diseases for which a larger than expected number of cases occurs in a short time within a geographic region are called <strong>epidemic disease<\/strong><strong>s<\/strong>. Influenza is a good example of a commonly epidemic disease. Incidence patterns of influenza tend to rise each winter in the northern hemisphere. These seasonal increases are expected, so it would not be accurate to say that influenza is epidemic every winter; however, some winters have an usually large number of seasonal influenza cases in particular regions, and such situations would qualify as epidemics (Figure\u00a02\u00a0and Figure\u00a03).<\/p>\n<div style=\"width: 711px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03165430\/OSC_Microbio_16_01_Flu.jpg\" alt=\"A graph of the percentage of emergency department visits for influenza-like illness. The X axis is times of the year and the Y axis is percent. The national baseline is near 2.5%. All yeas have a small peak in January and a larger peak February to April. 2007-2008 had the largest peak in February to April. 2008 \u2013 2009 had an additional peak from May to September.\" width=\"701\" height=\"448\" \/><\/p>\n<p class=\"wp-caption-text\">Figure\u00a02. The 2007\u20132008 influenza season in the United States saw much higher than normal numbers of visits to emergency departments for influenza-like symptoms as compared to the previous and the following years. (credit: modification of work by Centers for Disease Control and Prevention)<\/p>\n<\/div>\n<div style=\"width: 709px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03165433\/OSC_Microbio_16_01_Flu2.jpg\" alt=\"A graph of the percentage of all deaths due to influenza an pneumonia. The X axis is years and the Y axis is percent. The seasonal baseline fluctuates to a high in winter. The epidemic threshold is just a half percent higher than the baseline. The actual mortality fluctuates above and below both of these lines.\" width=\"699\" height=\"408\" \/><\/p>\n<p class=\"wp-caption-text\">Figure\u00a03. The seasonal epidemic threshold (blue curve) is set by the CDC-based data from the previous five years. When actual mortality rates exceed this threshold, a disease is considered to be epidemic. As this graph shows, pneumonia- and influenza-related mortality saw pronounced epidemics during the winters of 2003\u20132004, 2005, and 2008. (credit: modification of work by Centers for Disease Control and Prevention)<\/p>\n<\/div>\n<p>An epidemic disease signals the breakdown of an equilibrium in disease frequency, often resulting from some change in environmental conditions or in the population. In the case of influenza, the disruption can be due to antigenic shift or drift (see <a href=\".\/chapter\/virulence-factors-of-bacterial-and-viral-pathogens\/\" target=\"_blank\" rel=\"noopener\">Virulence Factors of Bacterial and Viral Pathogens<\/a>), which allows influenza virus strains to circumvent the acquired immunity of their human hosts.<\/p>\n<p>An epidemic that occurs on a worldwide scale is called a <strong>pandemic disease<\/strong>. For example, <strong>HIV\/AIDS<\/strong> is a pandemic disease and novel influenza virus strains often become pandemic.<\/p>\n<div class=\"textbox key-takeaways\">\n<h3>Think about It<\/h3>\n<ul>\n<li>Explain the difference between sporadic and endemic disease.<\/li>\n<li>Explain the difference between endemic and epidemic disease.<\/li>\n<\/ul>\n<\/div>\n<h2>Etiology<\/h2>\n<p>When studying an epidemic, an epidemiologist\u2019s first task is to determinate the cause of the disease, called the <strong>etiologic agent<\/strong> or <strong>causative agent<\/strong>. Connecting a disease to a specific pathogen can be challenging because of the extra effort typically required to demonstrate direct causation as opposed to a simple association. It is not enough to observe an association between a disease and a suspected pathogen; controlled experiments are needed to eliminate other possible causes. In addition, pathogens are typically difficult to detect when there is no immediate clue as to what is causing the outbreak. Signs and symptoms of disease are also commonly nonspecific, meaning that many different agents can give rise to the same set of signs and symptoms. This complicates diagnosis even when a causative agent is familiar to scientists.<\/p>\n<p>Robert <strong>Koch<\/strong> was the first scientist to specifically demonstrate the causative agent of a disease (anthrax) in the late 1800s. Koch developed four criteria, now known as <strong>Koch\u2019s postulates<\/strong>, which had to be met in order to positively link a disease with a pathogenic microbe. Without Koch\u2019s postulates, the <strong>Golden Age of Microbiology<\/strong> would not have occurred. Between 1876 and 1905, many common diseases were linked with their etiologic agents, including cholera, diphtheria, gonorrhea, meningitis, plague, syphilis, tetanus, and tuberculosis. Today, we use the <strong>molecular Koch\u2019s postulates<\/strong>, a variation of Koch\u2019s original postulates that can be used to establish a link between the disease state and virulence traits unique to a pathogenic strain of a microbe. Koch\u2019s original postulates and molecular Koch\u2019s postulates were described in more detail in <a href=\".\/chapter\/how-pathogens-cause-disease\/\" target=\"_blank\" rel=\"noopener\">How Pathogens Cause Disease<\/a>.<\/p>\n<div class=\"textbox key-takeaways\">\n<h3>Think about It<\/h3>\n<ul>\n<li>List some challenges to determining the causative agent of a disease outbreak.<\/li>\n<\/ul>\n<\/div>\n<h2>The Role of Public Health Organizations<\/h2>\n<p>The main national public health agency in the United States is the <strong>Centers for Disease Control and Prevention (CDC)<\/strong>, an agency of the Department of Health and Human Services. The CDC is charged with protecting the public from disease and injury. One way that the CDC carries out this mission is by overseeing the <strong>National Notifiable Disease Surveillance System<\/strong> (NNDSS) in cooperation with regional, state, and territorial public health departments. The NNDSS monitors diseases considered to be of public health importance on a national scale. Such diseases are called <strong>notifiable disease<\/strong><strong>s<\/strong> or <strong>reportable disease<\/strong><strong>s<\/strong> because all cases must be reported to the CDC. A physician treating a patient with a notifiable disease is legally required to submit a report on the case. Notifiable diseases include <strong>HIV<\/strong> infection, <strong>measles<\/strong>, <strong>West Nile virus<\/strong> infections, and many others. Some states have their own lists of notifiable diseases that include diseases beyond those on the CDC\u2019s list.<\/p>\n<p>Notifiable diseases are tracked by epidemiological studies and the data is used to inform health-care providers and the public about possible risks. The CDC publishes the <strong><em>Morbidity and Mortality Weekly Report<\/em> (<em>MMWR<\/em>)<\/strong>, which provides physicians and health-care workers with updates on public health issues and the latest data pertaining to notifiable diseases. Table 1\u00a0is an example of the kind of data contained in the <em>MMWR<\/em>.<\/p>\n<table id=\"fs-id1167660338485\" class=\"span-all\" summary=\"Table titled: Incidence of Four Notifiable Diseases in the United States, Week Ending January 2, 2016. Columns: Disease; Current Week (Jan 2, 2016); Median of Previous 52 weeks; Maximum of previous 52 weeks; Cumulative Cases 2015. Campylobacteriosis: 406; 869; 1,385; 46,618. Chlamydia trachomatis infection: 11,024; l28,562; 31,089; 1,425,303. Giardiasis: 115; 230; 335; 11,870. Gonorrhea: 3,207; 7,155; 8,283; 369,926.\">\n<thead>\n<tr>\n<th colspan=\"5\">Table 1. Incidence of Four Notifiable Diseases in the United States, Week Ending January 2, 2016<\/th>\n<\/tr>\n<tr valign=\"top\">\n<th>Disease<\/th>\n<th>Current Week (Jan 2, 2016)<\/th>\n<th>Median of Previous 52 Weeks<\/th>\n<th>Maximum of Previous 52 Weeks<\/th>\n<th>Cumulative Cases 2015<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr valign=\"top\">\n<td>Campylobacteriosis<\/td>\n<td>406<\/td>\n<td>869<\/td>\n<td>1,385<\/td>\n<td>46,618<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td><em>Chlamydia trachomatis<\/em> infection<\/td>\n<td>11,024<\/td>\n<td>28,562<\/td>\n<td>31,089<\/td>\n<td>1,425,303<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Giardiasis<\/td>\n<td>115<\/td>\n<td>230<\/td>\n<td>335<\/td>\n<td>11,870<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Gonorrhea<\/td>\n<td>3,207<\/td>\n<td>7,155<\/td>\n<td>8,283<\/td>\n<td>369,926<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<div class=\"textbox\">The current <a href=\"http:\/\/www.cdc.gov\/mmwr\/index.html\"><em>Morbidity and Mortality Weekly Report<\/em><\/a> is available online.<\/div>\n<div class=\"textbox key-takeaways\">\n<h3>\u00a0Think about It<\/h3>\n<ul>\n<li>Describe how health agencies obtain data about the incidence of diseases of public health importance.<\/li>\n<\/ul>\n<\/div>\n<div class=\"textbox key-takeaways\">\n<h3>Key Concepts and Summary<\/h3>\n<ul>\n<li><strong>Epidemiology<\/strong> is the science underlying public health.<\/li>\n<li><strong>Morbidity<\/strong> means being in a state of illness, whereas <strong>mortality<\/strong> refers to death; both <strong>morbidity rates<\/strong> and <strong>mortality rates<\/strong> are of interest to epidemiologists.<\/li>\n<li><strong>Incidence<\/strong> is the number of new cases (morbidity or mortality), usually expressed as a proportion, during a specified time period; <strong>prevalence<\/strong> is the total number affected in the population, again usually expressed as a proportion.<\/li>\n<li><strong>Sporadic diseases<\/strong> only occur rarely and largely without a geographic focus. <strong>Endemic diseases<\/strong> occur at a constant (and often low) level within a population. <strong>Epidemic diseases<\/strong> and <strong>pandemic diseases<\/strong> occur when an outbreak occurs on a significantly larger than expected level, either locally or globally, respectively.<\/li>\n<li><strong>Koch\u2019s postulates<\/strong> specify the procedure for confirming a particular pathogen as the etiologic agent of a particular disease. Koch\u2019s postulates have limitations in application if the microbe cannot be isolated and cultured or if there is no animal host for the microbe. In this case, molecular Koch\u2019s postulates would be utilized.<\/li>\n<li>In the United States, the <strong>Centers for Disease Control and Prevention<\/strong> monitors <strong>notifiable diseases<\/strong> and publishes weekly updates in the <em>Morbidity and Mortality Weekly Report.<\/em><\/li>\n<\/ul>\n<\/div>\n<div class=\"textbox exercises\">\n<h3>Matching<\/h3>\n<p>Match each term with its description.<\/p>\n<table id=\"fs-id1167662472243\" class=\"unnumbered unstyled\" summary=\"sporadic disease; endemic disease; pandemic disease; morbidity rate; mortality rate. A. the number of disease cases per 100,000 individuals. B. a disease in higher than expected numbers around the world. C. the number of deaths from a disease for every 10,000 individuals. D. a disease found occasionally in a region with cases occurring mainly in isolation from each other. E. a disease found regularly in a region.\">\n<tbody>\n<tr valign=\"top\">\n<td>___sporadic\u00a0disease<\/td>\n<td>A. the number of disease cases per 100,000 individuals<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>___endemic\u00a0disease<\/td>\n<td>B. a disease in higher than expected numbers around the world<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>___pandemic\u00a0disease<\/td>\n<td>C. the number of deaths from a disease for every 10,000 individuals<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>___morbidity\u00a0rate<\/td>\n<td>D. a disease found occasionally in a region with cases occurring mainly in isolation from each other<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>___mortality\u00a0rate<\/td>\n<td>E. a disease found regularly in a region<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q524314\">Show Answer<\/span><\/p>\n<div id=\"q524314\" class=\"hidden-answer\" style=\"display: none\">\n<ol>\n<li>(D) Sporadic disease is\u00a0a disease found occasionally in a region with cases occurring mainly in isolation from each other.<\/li>\n<li>(E) Endemic disease is\u00a0a disease found regularly in a region.<\/li>\n<li>(B) Pandemic disease is\u00a0a disease in higher than expected numbers around the world.<\/li>\n<li>(A) Morbidity rate is the number of disease cases per 100,000 individuals.<\/li>\n<li>(C) Mortality rate is\u00a0the number of deaths from a disease for every 10,000 individuals.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox exercises\">\n<h3>Fill in the Blank<\/h3>\n<p>The ________ collects data and conducts epidemiologic studies in the United States.<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q261307\">Show Answer<\/span><\/p>\n<div id=\"q261307\" class=\"hidden-answer\" style=\"display: none\">The <strong>CDC (Centers for Disease Control and Prevention)<\/strong> collects data and conducts epidemiologic studies in the United States.<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox key-takeaways\">\n<h3>Think about It<\/h3>\n<ol>\n<li>During an epidemic, why might the prevalence of a disease at a particular time not be equal to the sum of the incidences of the disease?<\/li>\n<li>In what publication would you find data on emerging\/reemerging diseases in the United States?<\/li>\n<li>Why might an epidemiological population in a state not be the same size as the number of people in a state? Use an example.<\/li>\n<\/ol>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-714\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>OpenStax Microbiology. <strong>Provided by<\/strong>: OpenStax CNX. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2\">http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section><hr class=\"before-footnotes clear\" \/><div class=\"footnotes\"><ol><li id=\"footnote-714-1\">H. Irene Hall, Qian An, Tian Tang, Ruiguang Song, Mi Chen, Timothy Green, and Jian Kang. \"Prevalence of Diagnosed and Undiagnosed HIV Infection\u2014United States, 2008\u20132012.\" <em>Morbidity and Mortality Weekly Report<\/em> 64, no. 24 (2015): 657\u2013662. <a href=\"#return-footnote-714-1\" class=\"return-footnote\" aria-label=\"Return to footnote 1\">&crarr;<\/a><\/li><li id=\"footnote-714-2\">Centers for Disease Control and Prevention. \"Diagnoses of HIV Infection in the United States and Dependent Areas, 2014.\" <em>HIV Surveillance Report<\/em> 26 (2015). <a href=\"#return-footnote-714-2\" class=\"return-footnote\" aria-label=\"Return to footnote 2\">&crarr;<\/a><\/li><li id=\"footnote-714-3\">Centers for Disease Control and Prevention. \"Tetanus Surveillance\u2014United States, 2001\u20132008.\" <em>Morbidity and Mortality Weekly Report<\/em> 60, no. 12 (2011): 365. <a href=\"#return-footnote-714-3\" class=\"return-footnote\" aria-label=\"Return to footnote 3\">&crarr;<\/a><\/li><li id=\"footnote-714-4\">Centers for Disease Control and Prevention. \"Plague in the United States.\" 2015. http:\/\/www.cdc.gov\/plague\/maps. Accessed June 1, 2016. <a href=\"#return-footnote-714-4\" class=\"return-footnote\" aria-label=\"Return to footnote 4\">&crarr;<\/a><\/li><\/ol><\/div>","protected":false},"author":17,"menu_order":2,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"OpenStax Microbiology\",\"author\":\"\",\"organization\":\"OpenStax CNX\",\"url\":\"http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2\"}]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-714","chapter","type-chapter","status-publish","hentry"],"part":707,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/chapters\/714","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":4,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/chapters\/714\/revisions"}],"predecessor-version":[{"id":2197,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/chapters\/714\/revisions\/2197"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/parts\/707"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/chapters\/714\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/wp\/v2\/media?parent=714"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/chapter-type?post=714"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/wp\/v2\/contributor?post=714"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/wp\/v2\/license?post=714"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}