Some new directions for research in epidemic models

Problems requiring further development of epidemic theory are discussed. One important area is the association between progressive disease and horizontally-transmitted agents, as in, e.g., the current epidemic of acquired immunodeficiency syndrome (AIDS) which has been linked to spread of a virus. Models of disease transmission must be combined with models of disease progression. More work is also needed to explain why the annual incidence of some diseases fluctuates dramatically, often in regular cycles, rather than stabilizing at a predictable level.