Why some infectious diseases are limited to small outbreaks and others become full-blown epidemics

In an important study forthcoming in the March 2006 issue of the American Naturalist, biologists from Yale University, University of Florida, and Dartmouth University explore the dynamics of pathogen survival and shed new light on a longstanding mystery: why some infectious diseases are limited to small outbreaks and others become full-blown epidemics.

"The capacity of a virus to propagate upon a novel host apparently is conditional on the recent experience of preceding generations," the authors say. "This is intrinsically interesting, suggesting a kind of complexity in pathogen population dynamics that has not been widely regarded."

The researchers observed viral populations on host bacteria, specifically situations where virus populations were sustained on the original hosts, but went extinct on the new hosts. Observing transmission rates, they found that viruses previously reared on an original host showed greater productivity on the new host than viruses previously reared on the new host.

"In this critical region, periodic exposure to native hosts allowed the viruses to survive on novel hosts, an unanticipated result," explain the authors.

The researchers infer that the mechanism behind this phenomenon may be the "host-legacy" effect. If this is the case, according to the authors, the total viral population experiencing the new environment is greater than previously expected, allowing for increased chances of adaptive evolution to the new host.