Crossing the species barrier

Crossing the species barrier requires several steps, the first being contact with the microbe - and travel today provides more potential exposure events with a wide range of other species or microbes in the environment than ever before. To cross successfully from one species to another, a virus must be able to find a cell type that it can infect (Webby et al., 2004). Even if it can replicate, it must be able to avoid or overcome host immune response, exit from the cell, and infect additional cells. Finally, it must be able to exit the host in such a way that it can be transmitted to other humans. For many animal viruses, humans are always or usually a dead-end host (e.g. hantaviruses causing hantavirus pulmonary syndrome), and are irrelevant in maintenance of the microbe in nature. The avian influenza A virus (H5N1) has not transmitted efficiently from person to person as of mid-2006, and a potential explanation is that the cells in the upper respiratory tract of humans do not have receptors to which the virus can bind. The virus can replicate efficiently only in the lower respiratory tract (alveolar cells) of humans, from which it is not transmitted easily by sneezing and coughing (Shinya et al., 2006).

RNA viruses have been prominent among microbes causing emerging infections. They are extremely mutable; many mutations can occur during replication, and

RNA viruses have few or no proofreading mechanisms (Webby et al., 2004). These characteristics generate viral diversity. Variants with any advantage in a particular host or under specific environmental conditions are selected and can amplify. These viruses can also generate diversity through recombination and reassortment.

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