Epidemiological investigations are carried out during outbreaks to determine the cause of the outbreak and put control measures to prevent new diseases and deaths. Outbreak investigations usually comprise several stages (which may differ in order depending on the outbreak), including:  

• Establish the existence of an outbreak after initial reports, for example, by a health worker or in surveillance data, of a rise in the number of disease cases. As discussed earlier, this would be achieved by comparing the observed and expected number of cases  

• Search for additional cases tthat may not yet have been reported, for example, by contacting health facilities and laboratories, or by asking infected cases or their contacts if they have come across other people with similar symptoms, and using carefully defined case definitions (this will be covered in sections 5 and 6)  

• Characterising the outbreak in terms of person (the type of people infected), place (where cases have occurred) and time (when people became ill). This step requires gathering, analyzing, and recording the age and sex of the infected people to describe the outbreak in terms of 'person.'

• Develop a hypothesis (theory) about the source of the outbreak and how the disease is transmitted. Test this hypothesis using preliminary data, or by conducting a further study such as survey interviews with infected individuals followed by a more comprehensive statistical analysis (details of more advanced studies and statistical analysis step are beyond the scope of this session). Several variations of this hypothesis may be formulated and tested if the first presumptions do not appear to be correct.

• Design and put in place strategies to control the outbreak. This will be discussed further in section 6.