So far we have seen the team at the World Health Organisation (WHO) going about their usual daily business of checking reports from around the world. The death of the person from the plane in Hanoi caught their attention because some of the doctors and nurses who treated the patient had become ill. These are otherwise healthy people who would be aware of the risks of disease. At this point epidemiologists went to work. Epidemiologists study the incidence, distribution and control of diseases.