Disease outbreak happens when there are more cases of an infectious disease than expected in a community or geographical area. Diseases can spread directly from person to person or may spill over from animals to humans as a result of zoonotic transmission or by insect vectors. Disease outbreaks can be caused by a wide range of pathogens, from familiar diseases such as plague and diphtheria to new and re-emerging infectious diseases including SARS-CoV-1, Ebola, Lassa fever and Zika.
A key part of an outbreak investigation involves the use of methods from descriptive and, where appropriate, analytical epidemiology. Descriptive epidemiology shows the occurrence of disease in terms of time, place and affected person; it allows us to assess whether or not a cluster of cases (two or more cases of the same disease at the same time) occurs. Analytical epidemiology, on the other hand, provides information about the centre, source and mode of transmission, allowing us to make specific hypotheses about the cause and mechanisms of an outbreak.
When applied to a disease outbreak, these activities shift over the phases of an outbreak: Surveillance and preparedness (“Detect and investigate suspected cases”), early response (“Determine mode of transmission and perform contact tracing”) and recovery (“Evaluate intervention impact and recovery”) as shown in Figure 1. Many of the same activities are applicable to all types of biological events — whether they are accidental, deliberate or unknown — but several activities are more relevant for certain types of events than others (e.g., “Consider request for investigation into potential breach of the Biological Weapons Convention”; “Ensure chain of custody of samples”).