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h virus in Malaysia. For example, bats are considered to be the reservoirs of the Nipah virus, and thus when people decided to plant fruit orchards around piggeries, the bats became attracted to the orchards and caused a spillover which infected the pigs. In turn, people working with the infected pigs became exposed to the virus and this caused an increase in animal virus transmission.

Host barriers are the second variable examined in the article and proven to affect the mechanism of host switching. In order for transmission to be a possibility, a virus has to be able to infect cells of a new host. Yet, this process can be delayed at various levels such as receptor binding, entry into the cell, genome replication or gene expression. Based on these multiple host barriers, the virus would have to undergo changes to be able to overcome everything thus increasing the difficulty of transmission. Also this article states that innate antiviral responses from host cells and apolipoprotein B-editing catalytic polypeptide proteins (APOBEC) further impede the risk of infection by blocking infection to subsequent cells. In examining evolutionary relatedness, species that are closely related to one another have an increased likelihood of viral host switching as viewed between chimpanzees and humans, thus resulting in the establishment of HIV. On the other hand, due to relatedness, certain limitations based on cross-immunity to related pathogens and innate immune resistances to related viral groups arise. Another aspect to host barriers is the physical entry of the virus into the cell. Upon entry there are host glycans or lectins which bind to the virus particles to prevent infection. Also, lack of neuraminidase proteins, used in the process of egress, cause viral inactivation which further aids in the prevention of transmission and emergence. Because viruses are specific to their appropriate host, they are also specific to the various receptors in which they bind to the host cell. For example, the HIV virus binds specifically to CD4 host receptors whereas avian viruses recognize sialic acids found on host cells. Aside from receptor binding, there are also intracellular restrictions which decrease viral transmission. For example, interferon responses are found to be host specific and thus more likely to protect cells against viruses. This can be observed by alpha and beta interferons which restrict the murine norovirus from entering the host cell.

The author further examined the host ranges of viruses and whether or not they were a factor in host switching. It was conjectured that preexisting host ranges influence the ability of a virus to be established in a new host. Viruses were classified as either generalist, infecting many different hosts, or specialist, infecting only a few related hosts. The expectation was that generalist viruses would show a greater likelihood of shifting to additional hosts whereas specialist viruses would be more inclined to restrictions of host switching. However, looking at the data in Table 1, it became apparent that both generalist and specialist viruses have transmitted successfully into new hosts thus diminishing the overall generalization previously made.

The last variable under review for affecting the mechanism of host switching is the viral evolutionary mechanisms which consist of viral fitness trade-offs, modes of virus transmission, recombination and reassortm