Outbreak is a term used to describe the occurrence of cases of disease in excess of what would be expected in a community, geographical area or season. The disease may be caused by infectious agents that spread directly from person to person or indirectly through contact with animal reservoirs, environmental sources, zoonotic organisms and insect vectors. Infectious diseases that can be caused by outbreaks include plague, diphtheria, Ebola, malaria, cholera, measles, SARS, influenza and chikungunya.

Outbreak investigations are complex and require rigorous data collection and analysis. A good understanding of outbreak investigation methodologies is essential for investigators to collect and report accurate information.

The goal of an outbreak investigation is to identify the cause(s) of the outbreak and to take steps to prevent future outbreaks. The basic epidemiological tools for investigation are case-control analyses, matched cohort studies and surveillance (e.g., re-evaluation of controls).

Case counts are updated weekly during the course of an outbreak investigation. For the purpose of outbreak investigations, the number of cases includes all persons who have been diagnosed with symptoms consistent with C. difficile infection. The total case count is also available in CDC and FDA Advisories.

Descriptive epidemiology is a standard component of an outbreak investigation and allows investigators to identify what food or drink might be responsible for the outbreak. For example, if an outbreak of Salmonella Agbeni illnesses is linked to a cake mix, investigators will list all foods and drinks served during the event and then divide the group into those who consumed the cake mix and those who did not. The attack rate is calculated and the food or drink items with the highest attack rates are identified (Center for Disease Control and Prevention 2008).