Outbreak refers to a higher-than-expected frequency of disease in a population. Thousands of outbreaks have been reported and investigated over the past two centuries, including cholera, smallpox, malaria, influenza, SARS, AIDS, measles, polio, foodborne illness, and others (see Disease Outbreak News).
Epidemiologists use methods of descriptive and, when appropriate, analytical epidemiology to study an outbreak. The goal is to identify the source(s) of an epidemic so that it can be stopped or prevented.
In general, an outbreak is suspected when data from several cases display common characteristics: occurrence of the same disease in the same time and place; a higher-than-expected number of cases in a given area over a set period of time (e.g., days, weeks, months or years); and the appearance of similar symptoms in patients with the same diagnosis. The investigation may involve sampling and testing to confirm or rule out various hypotheses, and may also include monitoring of the spread of the outbreak and the development of control strategies (e.g., isolation of affected populations, contact tracing).
The GOAL website captures 229 outbreak response activities that fall into six broad categories: epidemiology & lab analysis; healthcare and infection, prevention, and control (IPC); governance and coordination; humanitaria assistance; and logistics and support. Most of these activities can be applied to any type of outbreak, whether natural, accidental, or deliberate (e.g., identifying a possible breach of the Biological Weapons Convention and ensuring chain of custody of samples). Interestingly, when grouped by outbreak phase, it appears that the vast majority of activities are needed at the earliest stages of response (“Determine mode of transmission”; “Perform contact tracing activities”). This suggests that if basic epidemic control activities were incorporated into routine public health practice, many different types of outbreaks could be mitigated or averted (Fig. 1).