A disease outbreak is a group of cases in a community or region that appear to be caused by the same infectious agent. An outbreak investigation involves using methods of descriptive and, where appropriate, analytical epidemiology to detect the infection’s source and mode of transmission. An effective outbreak investigation requires the involvement of multiple disciplines and agencies.
In order to determine whether an outbreak is occurring, health officials must first compare the number of cases in the area to the expected number of cases based on historical data (e.g., data from disease registries and hospital discharge records). However, the comparison must be made with caution because factors such as variations in reporting procedures, changes in population size, or improved diagnostic techniques could influence the results and lead to false-positive alarms.
The next step is to identify clusters by analyzing the distribution of cases in time and space. The identification of clusters is an important part of the process because it allows health investigators to focus on the areas most likely to contain the cause of the outbreak. Clusters can be identified by the occurrence of two or more cases that have similar characteristics, such as symptoms, time of onset, and geographic location.
A final step is to implement control and prevention measures. Depending on the type of outbreak, this may include removing suspect foods from the market or closing restaurants; educating the public about food safety; or changing practices in facilities or programs that handle risk-bearing products (e.g., food processing plants, public pools, and schools). In addition to these general control measures, in Virginia, Section 32.1-37 requires certain facilities and programs to report suspected outbreaks to the local health department.