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Microbiology and Molecular Biology Reviews, September 2008, p. 457-470, Vol. 72, No. 3
1092-2172/08/$08.00+0     doi:10.1128/MMBR.00004-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Cross-Species Virus Transmission and the Emergence of New Epidemic Diseases

Baker Institute for Animal Health, College of Veterinary Medicine, Cornell University, Ithaca, New York 14853,¹ Center for Infectious Disease Dynamics, Department of Biology, The Pennsylvania State University, University Park, Pennsylvania 16802,² National Institute of Allergy and Infectious Diseases, National Institutes of Health, Building 31, Room 7A-03, Bethesda, Maryland 20892,³ Virology Branch, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Room 4103, 6610 Rockledge Dr., Bethesda, Maryland 20892-7630,⁴ Graduate School of Public Health, University of Pittsburgh, Room A624, 130 DeSoto Street, Pittsburgh, Pennsylvania 15261,⁵ Arthropod-Borne and Infectious Diseases Laboratory, Department of Microbiology, Immunology and Pathology, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, Colorado 80523,⁶ Food Animal Health Research Program, Ohio Agricultural Research and Development Center, The Ohio State University, 1680 Madison Ave., Wooster, Ohio 44691,⁷ Consortium for Conservation Medicine, 460 West 34th Street, New York, New York 10001⁸

Summary: Host range is a viral property reflecting natural hosts that are infected either as part of a principal transmission cycle or, less commonly, as "spillover" infections into alternative hosts. Rarely, viruses gain the ability to spread efficiently within a new host that was not previously exposed or susceptible. These transfers involve either increased exposure or the acquisition of variations that allow them to overcome barriers to infection of the new hosts. In these cases, devastating outbreaks can result. Steps involved in transfers of viruses to new hosts include contact between the virus and the host, infection of an initial individual leading to amplification and an outbreak, and the generation within the original or new host of viral variants that have the ability to spread efficiently between individuals in populations of the new host. Here we review what is known about host switching leading to viral emergence from known examples, considering the evolutionary mechanisms, virus-host interactions, host range barriers to infection, and processes that allow efficient host-to-host transmission in the new host population.