Parasite Diversity and Parasite Coexistence

These papers deal with different aspects of understanding how hosts cope with the diversity of antigenic challenges that pathogens provide. These comments examine common threads underlying three of the presentations and describe some recent work on the more general problem of the diversity of pathogens that any host population can sustain.

The work of Martin Nowak and his colleagues at Oxford, Imperial College and Amsterdam University epitomizes the challenges that mathematical models present to empirical epidemiologists. As with other recent work on the mathematics of the immune system and infection with HIV (McLean 1993), the work suggests alternative interpretations of epidemiological data and has stimulated the collection (and analysis) of data not normally collected by immunologists and clinicians. At the heart of the Nowak model is the interaction between the diversity of HIV quasi-species in individual patients and the ability of the host to produce a sufficient diversity of antigens to cope with this. At a crucial level of quasi-species diversity, the diversity threshold, the immune system is overwhelmed, CD4 counts decline precipitously and the patient succumbs to full-blown AIDS. The length of time until this occurs is dependent upon a variety of factors, but most importantly upon the replication and mutation rate of the virus, and upon the host's ability to mount an efficient and diverse immune response. My main questions are about this diversity threshold; the diversity levels presented in the model seem much higher than the diversity levels observed in individually infected HIV/AIDS patients.