Occurrence of a Disease

Occurrence of a Disease

Outbreak is a term used in epidemiology to describe an occurrence of disease greater than would otherwise be expected at a particular time and place. It may affect a small and localized group or impact thousands of people across an entire continent. Two linked cases of a rare infectious disease may be sufficient to constitute an outbreak. Outbreaks may also refer to endemics that affect a particular place or group, epidemics that affect a region in a country or a group of countries, and pandemics that describe global disease outbreaks .

1918 Flu Victims

With masks over their faces, members of the American Red Cross remove a victim of the Spanish Flu from a house at Etzel and Page Avenues, St. Louis, Missouri.

The epidemiology profession has developed a number of widely accepted steps when investigating disease outbreaks. As described by the Centers for Disease Control and Prevention, these include the following:

1. Verify the diagnosis related to the outbreak.
2. Identify the existence of the outbreak (if the group of ill persons is normal for the time of year, geographic area, etc. ).
3. Create a case definition to define who/what is included as a case.
4. Map the spread of the outbreak.
5. Develop a hypothesis (if there appears to be a cause for the outbreak).
6. Study hypothesis (collect data and perform analysis).
7. Refine hypothesis and carry out further study.
8. Develop and implement control and prevention systems.
9. Release findings to greater communities.

There are several outbreak patterns that can be useful in identifying the transmission method or source and predicting the future rate of infection.

1. Common source – All victims acquire the infection from the same source (e.g. a contaminated water supply).
2. Continuous source – Common source outbreak where the exposure occurs over multiple incubation periods.
3. Point source – Common source outbreak where the exposure occurs in less than one incubation period.
4. Propagated – Transmission occurs from person to person.

Each has a distinctive epidemic curve, or histogram of case infections and deaths.

Outbreaks can also be:

1. Behavioral risk related (e.g. sexually transmitted diseases, increased risk due to malnutrition)
2. Zoonotic – The infectious agent is endemic to an animal population.