Outbreak investigation: the need for 'quick and clean' epidemiology.

Epidemiology investigations of outbreaks of infectious disease have to be carried out rapidly but must be methodologically sound. In 14 of 25 consecutive Salmonella outbreaks in Wales from 1986 to 1990 case-control or cohort studies were undertaken. Food vehicles of infection were identified by statistically significant associations with cases in 13 of the 14, including three outbreaks where there were less than 10 cases. The particular problems of epidemiological field investigations are discussed.