Infectious diseases have plagued mankind for as long as we have been around. Infectious diseases are caused by pathogenic microorganisms. There are four types of microorganisms that affect human health: viruses, bacteria, fungi, and parasites. Viruses are the smallest and simplest microorganism listed. They are single-celled organisms. Examples of viral infections include the influenza virus, Ebola virus, Zika virus, and smallpox. Bacteria are also single-celled organisms. Bacterial infections include salmonella, E. coli, and Staphylococcus aureus. Fungi are the next biggest microorganism. Fungi can be single or multicellular. There are over one hundred thousand fungi, but very few can cause disease in humans. An example of this Trichophyton rubrum. The last type of pathogenic microorganism are parasites. They are the largest, having the ability to be several feet in length. Parasites affect people in tropical and subtropical environments the most. Examples of parasites include ringworm and hookworm. (S. Zorich, personal communication, February 17, 2018).

Infectious diseases can be spread in direct and indirect ways. Direct transmission routes can include skin-to-skin contact, direct spraying of respiratory fluids (i.e. coughing directing on someone). Indirect transmission routes are foodborne, waterborne, airborne (when the particles are suspended in the air or travel), and vector borne (S. Zorich, personal communication, February 17, 2018).

There are a few ways to classify diseases. The first is the level of disease. An endemic is the amount of disease that is typically present in an area. Following that there is an epidemic, which is when the disease frequency is higher than endemic levels in a defined area. The last is pandemic, which is an epidemic that has great geographic reach. It is an international epidemic and affects a large number of people (S. Zorich, personal communication, February 19, 2018).

According to WHO, disease outbreaks are when the frequency of a particular disease exceeds its normal expectancy in an area (WHO, Disease Outbreaks). Outbreaks can happen in single communities or affect multiple countries at a time. They can also last days to years (WedMD). Defining how many cases are needed to cause an outbreak can be difficult. For some diseases, a single case can constitute an outbreak. These diseases are usually emerging or re-emerging. An emerging disease is one that is one that has an increasing geographic range, increased incidence, or one that has not shown up in a population until an outbreak occurs (Baylor College of Medicine). New diseases are not ones that are rare. In fact, since the 1970s there have been roughly forty new disease discoveries. Some of these can include:

• – SARS
• – MERS
• – Ebola
• – Zika
• – Avian flu
• – Swine flu

There are many reasons why an outbreak can occur. Outbreaks of disease can be caused by (Wikipedia, 2018):

• – Previously unknown or undetected agents
• – Known agents that have spread to new locations
• – Genetic drift & genetic shift
• – Climate and weather patterns (i.e. increasing range of mosquito-borne illnesses)
• – Human travel patterns (i.e. SARS epidemic)
• – The breakdown of public health and government (i.e. Zimbabwe’s cholera epidemic)
• – Urbanization – people are more likely to live in densely populated areas, making disease transmission easy
• – A change in human susceptibility (i.e. measles outbreaks)
• – Bioterrorism attacks – the deliberate introduction of a disease with terrorist purposes (i.e. anthrax, smallpox)