Real populations are not homogeneous and the members of different subgroups do not mix randomly with all other individuals. Thus a population has in it subgroups that differ in ethnicity, religion and socio-economic status and these differences influence the rates of transmission of diseases. Two major ways are, the extent of mixing between subgroups as compared with within subgroup mixing, and the probability of transmission per contact may differ because of particular group practices or customs, for example, in food preparation. Population subgroups can also be defined by geographic location and such subroups intersect with subgroups defined by socio-economic and ethnic status. We defer consideration of defining geographic subgroups to the next major section. Furthermore, the subgoups relevant for transmission of one disease may differ from those important in the transmission of a different disease. Consequently, one has to examine the issue of definition of important subgroups for each particular disease.