Recent studies have shown that the relatively short period of exposure of human populations to malaria has left in its wake a wide range of genetic diversity. And there is growing evidence that other infectious agents have, or are, having the same effect. By integrating further studies of human populations with genetic analyses of susceptibility to murine malaria it should now be possible to determine some of the mechanisms involved in the variation of susceptibility to infectious disease, information which may have important practical implications for both the diagnosis and better management of these conditions.