A disease outbreak is a group of cases of disease that are larger than expected in a community or geographical area and that cluster together. They may be caused by infectious agents that spread directly from person to person, from an animal reservoir or other environmental source, or from inanimate objects such as insect vectors. Human behaviours nearly always contribute to the spread of diseases, and their impact can be devastating on societies and economies.
Outbreak investigations are essential for understanding the nature of disease outbreaks and for their subsequent control. These investigations should be conducted in a systematic and scientific manner (Ellis et al., 2003).
Investigations typically start with the collection of information from a wide range of formal and informal sources. This includes the local or regional Ministries of Health, WHO Country and Regional offices, military and civilian laboratories, academic institutions and WHO collaborating centres, and the Global Public Health Intelligence Network. The latter is a secure, Internet-based early warning system that continuously searches international media sources for reports of suspected disease or public health events.
An important aspect of any outbreak investigation is the determination of the mode(s) of transmission and the identification of the etiology (cause). A case-control study should be the preferred method of determining the etiology in an outbreak investigation under three circumstances: