Many pathogens are able to infect multiple species within a community and are commonly transmitted across species. Cross-species transmission is often associated with pathogen emergence and therefore has been considered as a negative factor for humans, wildlife, and species of agricultural importance. Many pathogens like malaria, Lyme disease or West Nile encephalitis that infect multiple hosts are commonly transmitted by vectors, and their transmission rate is often thought to depend on the proportion of hosts or vectors infected (i.e. frequency dependence). A study, to appear in the July 2005 issue of The American Naturalist, by Volker H. W. Rudolf and Janis Antonovics provides an important conceptual counterexample to the idea that sharing pathogens necessarily affects host populations negatively. They show that if there is frequency-dependent transmission, a host can be rescued from pathogen-mediated extinction by the presence of a second host with which it shares a pathogen. Sharing a pathogen among one or more alternative species may actually be limiting the spread of the pathogen in any one particular host population. Thus, an epidemic on one species may result because of loss or reduction in the abundance of species in the community, without any numerical or genetic change in the host or pathogen. Studying only the target species or the abiotic environment would not reveal the cause of the epidemic. Their results have not only important consequences for understanding the role of pathogens in species interactions but also provide a potential mechanism linking host biodiversity and ecosystem functioning.
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Volker H. W. Rudolf and Janis Antonovics, "Species coexistence and pathogens with frequency-dependent transmission" 165:7 July 2005.