This background lesson provides several working definitions of epidemiology—the basic science of public health; an introduction to the different categories of epidemiology and types of epidemiological studies; and an overview of the disease transmission cycle. First, to set the stage, consider the three incidents that follow, stepping into the shoes of the public health officer who received the initial report and asking yourself the question, “What do I do now?” Some of these examples made national news and may be familiar to you.

First Incident

In March 1985, a nurse epidemiologist in a county health department noted, while reviewing surveillance data, three cases in a single month of hepatitis B of unusual origin. Hepatitis B, or serum hepatitis, is transmitted through sexual contact and by exposure to infected bodily fluids, but these three patients did not seem to have the usual risk factors. All three people did, however, indicate having received injections at the same health care facility.

The nurse’s immediate questions were: “Is this a coincidence? Did these three cases occur by chance or is there a link?” In this instance, the nurse decided to pursue an investigation.

Second Incident

At 8:30 in the morning on August 2, 1976, Dr. Robert B. Craven of Centers for Disease Control’s (CDC) Viral Diseases Division received a call from a nurse at a Veterans’ Hospital in Philadelphia, Pennsylvania. The nurse reported two cases of severe respiratory illness, one of which had been fatal. Both people had attended the annual American Legion Convention held July 21-24. By the evening of August 2, 71 more of the people attending the convention had the same illness, with symptoms of acute onset of fever, chills, headache, malaise, dry cough, and myalgia. Further conversations with local and state public health officials revealed that between July 26 and August 2, 18 conventioneers had died. Deaths were due primarily to pneumonia.

An intense investigation began immediately. The incident became known as the first outbreak of Legionnaires’ disease and led to the discovery of the gram-negative pathogen, Legionnella pneumophila.

Third Incident

On October 30, 1989, a New Mexico physician notified the state’s health department of three patients with marked peripheral eosinophilia and severe myalgia. All three patients had been taking oral preparations of L-tryptophan, a nonprescription drug sold as a dietary supplement in health food stores. Despite extensive clinical evaluation and testing, the illness could not be identified.

An investigation followed and resulted in the characterization of eosinophilia-myalgia syndrome, EMS. The investigation implicated a vehicle for exposure—L-tryptophan dietary supplements—before a suspected agent was identified, and the product was taken off the market. Eventually, the problem was traced to a contaminant that had been introduced by changes in the production process at a single manufacturing facility.

These three examples illustrate some of the key reasons for needing applied, or field, epidemiology:

• They were unexpected.
• They demanded a response.
• The investigators had to go out into the field to solve the problem.

Epidemiology Defined

A definition of epidemiology is “the study of the distribution and determinants of health-related states in specified populations, and the application of this study to control health problems.” A look at the key words will help illuminate the meaning:

• Epidemiology is the basic science of public health. It’s a highly quantitative discipline based on principles of statistics and research methodologies.
• Distribution Epidemiologists study the distribution of frequencies and patterns of health events within groups in a population. To do this, they use descriptive epidemiology, which characterizes health events in terms of time, place, and person
• Determinant Epidemiologists also attempt to search for causes or factors that are associated with increased risk or probability of disease. This type of epidemiology, where we move from questions of “who,” “what,” “where,” and “when” and start trying to answer “how” and “why,” is referred to as analytical epidemiology

A comparison between the practice of public health and the more familiar practice of health care helps in describing epidemiology. First, where health care practitioners collect data on an individual patient by taking a medical history and conducting a physical exam, epidemiologists collect data about an entire population through surveillance systems or descriptive epidemiological studies. The health care practitioner uses his or her data to make a differential diagnosis. The epidemiologist’s data is used to generate hypotheses about the relationships between exposure and disease. Both disciplines then test the hypotheses, the health care practitioner by conducting additional diagnostic studies or tests, the epidemiologist by conducting analytical studies such as cohort or case-control studies. The final step is to take action. The health care practitioner prescribes medical treatment, and the epidemiologist, some form of community intervention to end the health problem and prevent its recurrence.

One way to sum up the task of epidemiologists is to say that they “count things.” Basically, epidemiologists count cases of disease or injury, define the affected population, and then compute rates of disease or injury in that population. Then they compare these rates with those found in other populations and make inferences regarding the patterns of disease to determine whether a problem exists.

For example, in the hepatitis B example earlier, you might ask: Is the rate of disease among people with no know risk factors greater than we would expect? Is the pattern or distribution of the cases suspicious? Once a problem has been identified, the data are used to determine the cause of the health problem; the modes of transmission; any factors that are related to susceptibility, exposure, or risk; and any potential environmental determinants.

Epidemiological Studies

As mentioned earlier, epidemiologists used several different types of studies. Simply speaking, these can be classified as either experimental, where the epidemiologists have control over the circumstances from the start, or observational, where they do not. Vaccine efficacy trials are a good example of experimental studies because investigators control who gets the vaccine and who doesn’t. Observational studies can be further subdivided into descriptive and analytical studies. In a descriptive study, the epidemiologist collects information to characterize and summarize the health event or problem. In an analytical study, the epidemiologist relies on comparisons between groups to determine the role of various risk factors in causing the problem. Descriptive epidemiology is the most basic of these categories and is fundamental to the work of an epidemiologists.

Another way of comparing descriptive and analytical epidemiology is to say that in the descriptive process, we are concerned with “person” (Who was affected?), “place” (Where were they affected?), and time (When were they affected?). Once we know the answers to these questions, we can enter the realm of analytical epidemiology and ask how and why these people were affected.

Disease Transmission

In talking about epidemiology, it is important to review how outbreaks occur. First, we’ll look at three commonly used, and often misunderstood, terms: “epidemic,” “outbreak,” and “cluster.” An epidemic is the occurrence of more cases of disease than would normally be expected in a specific place or group of people over a given period of time. To an epidemiologist, “outbreak” means basically the same thing. In the public’s mind, however, “epidemic” has a far more serious connotation than “outbreak.” For this reason, “outbreak” is often used to avoid sensationalism. The third term, “cluster,” is occasionally used, incorrectly, in place of “epidemic” or “outbreak.” A cluster is a group of cases in a specific time and place that may or may not be greater than the expected rate. Often the aim of investigating clusters is to determine the baseline rate of disease for that time and place. Two other terms you will come across are “endemic,” meaning a high background rate of disease, and “pandemic,” meaning very widespread, often global, disease.

For an outbreak, or epidemic, to occur, the basic elements of disease causation and an adequate chain of transmission must be present. Disease occurs when an outside agent capable of causing the disease meets a host that is vulnerable to the agent in an environment that allows the agent and host to interact. Then, given a chain of transmission from one host to another and a suitable mode of spread, an outbreak can develop. These basic concepts help guide the selection of public health strategies to prevent health problems. Depending on which approach might be most effective, we might direct efforts at the specific agent (e.g., guinea worm), host (e.g., immunization to prevent measles), or environment (e.g., sanitation improvements to prevent salmonella). We can also target a specific point in the chain of transmission. This was the response in the E.coli outbreak in Washington State in the early 1990s, when health officials called for the thorough cooking of hamburgers to interrupt transmission of the bacterium.

The host is the person, or in a more generic definition, the organism, that is susceptible to the effect of the agent. The status of the host is quite important and is generally classifiable as susceptible, immune, or infected. Finally, and also quite important, is that the host’s response to exposure can vary widely, from showing no effect to manifesting illness.

The environment is the conditions or influences that are not part of either the agent or the host, but that influence their interaction. A wide variety of factors, including physical, climatologic, biologic, social, and economic conditions, can come into play. For instance, in a study of motor vehicle injuries, the agent (mechanical energy) and the host (driver) could be affected by the topography, the weather, and the actions of other drivers. In many infectious disease outbreaks, social and economic conditions cause overcrowding and lead to high levels of exposure.

Agent, host, and environment alone are not sufficient to cause an epidemic; an adequate chain of transmission must be present. This process requires a source for the agent, a portal of exit, a mode of transmission, and a portal of entry. The first element, the source for the agent, is often the place where the agent originates, where it lives, grows, and multiplies, but this is not always the case. The agent that causes botulism (Clostridium botulinum), for example, originates in soil, but the source of most botulism infections is improperly canned food containing C. botulinum spores.

The second element, a portal of exit, is a pathway by which the agent can leave the source. This pathway is usually related to the place where the agent is localized. For instance, the agents causing tuberculosis and the flu are released through the respiratory tract, whereas agents for many stomach ailments are released through the digestive tract. Agents found in the blood, such as hepatitis B and HIV, can be released through cuts or needles.

Once the agent leaves the source, a mode of transmission, or means of carrying it to the host, is needed. This can happen in a number of ways, some of which are direct and some indirect. Direct transmission includes contact with soil or plants as well as contact between people. In indirect transmission, the agent can be airborne, vector borne, or vehicle borne. In airborne transmission, the agent is carried from the source to the host suspended in air particles. Vector-borne diseases are transmitted indirectly by a live carrier, usually an arthropod, such as mosquitos, fleas, or ticks. Vehicle-borne diseases are carried by inanimate objects, such as food or water, blood, or items like handkerchiefs, bedding, and surgical instruments.

Finally, there must be a pathway into the host, a portal of entry, that gives the agent access to tissue where it can multiply or act. Often the agent enters the host in the same way that it left the source. This is the case with the flu virus, which leaves the source through the respiratory tract and enters a new host through the respiratory tract.

The Chain of Infection Model

This model explains the spread of a communicable disease from one host (or person) to another. The basic idea represented in the chain of infection is that individuals can break the chain (reduce the risk) at any point, thus the spread of the disease can be stopped.

Now look at Table 1 carefully again. There are two sets of components that have some similar preventive measures. As a way of helping yourself become familiar with this chart see if you can spot which these are.

The portal of entry and exit both involve preventive measures such as hand washing, condoms, hair nets and insect repellents, while the human reservoir and transmission measures both involve isolation. Be sure you have a clear picture of the definition and prevention of each element before you continue.

With the application of such information, health education can help to create programs that are aimed at breaking the chain and reducing the risks of infection in other people.

According to the communicable disease model, the role of health education and health promotion in reducing the occurrence and transmission of diseases can be brought about by specific actions.

Think of tuberculosis (TB), malaria and intestinal infections as examples, and then answer the following questions:

1. Note one way to reduce the susceptibility of hosts.
2. Note one way to destroy infectious agents.
3. Note one way to reduce the contact between the host and the agent.
4. Note one way to modify the environment so that it is not conducive for disease transmission.

You may have answered with the following examples:

1. Good nutrition will build a person’s defenses against infection and reduce their risk of developing TB.
2. Cooking food properly destroys infectious agents that could cause intestinal infections.
3. Wearing a mask or holding your hand in front of your mouth while coughing will reduce the contact between the agent that causes TB, and other human beings.
4. Drying swampy and marshy areas will make the environment less easy for mosquitoes to breed in and therefore reduce the incidence of malaria.

Health risk reduction for non-communicable diseases

Both the chain of infection and communicable disease models are helpful in trying to prevent disease caused by an infectious agent. However, these models are not applicable to non-communicable diseases, which include many of the chronic diseases such as heart disease and cancers. Most of these diseases become apparent in people over a period of time and are not caused by a single factor, but by a combination of factors. The concept of ‘caused by many factors’ is often called the multi causation disease model. For example, it is known that heart disease is most likely to be a problem for individuals who are older, who smoke, who do not exercise, who are overweight, who have high blood pressure, who have high blood cholesterol and who have a family history of heart disease.

Note that within the list of factors you have just read there are both modifiable and non-modifiable risk factors.