ABSTRACT
One of the most challenging tasks of infectious diseases epidemiology is to try to decide if a disease is infectious or not. This chapter looks at some methods devised to investigate this problem. The most common method is to look for clusters in space and/or time, but an example of an ecological study is also given. Also, one example is given of the converse: searching for a disease for a newly found microbe. The general idea behind epidemiological studies that aim to find out if a disease is infectious is the following: if a disease can be transmitted from one person to another, then cases should tend to cluster in space and in time. For most diseases with a suspected infectious aetiology the research will be arduous: long incubation times, low transmission risks and perhaps genetic influences on disease expression are all factors that make the epidemiology of an infection difficult to elucidate.
