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ent and viral intermediates. It has been speculated in the article that cross-species transmission is more common in rapidly evolving viruses. This means the greater the rate of variation, the more likely a virus is able to adapt to a new host and undergo transmission. Because RNA viruses lack proofreading mechanisms and contain large viral populations, they are more likely to undergo evolution and transmit within a new host. Yet, there is evidence that some RNA viruses have developed host specialization and that rates of variation of DNA viruses should not be underestimated in comparison to RNA viruses. Because viruses are able to undergo various mutations, this increases their ability to infect new hosts but in the long run reduces their fitness in the donor host. By observing Figures 2 and 3 in the article, this is deemed fitness trade-off. However, not all mutations cause a reduction in fitness trade-off for there are a few advantageous ones that increase fitness. Also, when only a few adaptive mutations are required between donor host and recipient host, transmission becomes more efficient. As far as emergence and successful host transfer are concerned, the modes of virus transmission cause limitations. For example, if viruses are not able to survive between donor, recipient and or vector hosts, then emergence becomes a challenge. Furthermore, different pathways of transmission either by droplet spread, sexual inoculation and or fecal-oral represent challenges in accommodating various hosts. Recombination and reassortment make viruses more susceptible to genetic changes that are beneficial in the long run. In comparing RNA and DNA viruses, the possibility of recombination varies but in observing the retrovirus HIV, there is a high rate of recombination which may correlate to its effective emergence. Figure 5 examines possible roles of recombination in the HIV virus as well as its origin from other primates. Another example of a recombination virus can be viewed in SARS CoV which most likely arose from a combination of a CoV virus and another bat virus before infecting human hosts. Once a virus has been able to switch to a new host, it further uses recombination and reassortment in the process of adaptation. Lastly, there have been cases in which viral intermediates with lower fitness were required in the process of successful transmission. Aside from viruses adapting to their new hosts they also have to optimize their intensity in the host cells at the same time evading any immune responses. By being able to detect viruses that do not spread efficiently, there would be a greater chance of controlling epidemic outbreaks.

Reading through this scientific journal, the main purpose of the article is to present the audience with information on how viruses are able to cross species and bring about new epidemic diseases. The authors mostly define the barriers that a virus has to overcome in order to obtain successful transmission and overall emergence by organizing the information into various subheadings. For example, in the section Environmental and Demographic Barriers to Host Switching, the authors compare human actions such as travel or trade and link these to viral host switching and the potential for infection. In the next section, they focus on the host and the barriers that the virus has to overcome in order to give rise to efficient transmission. Basically the virus has to be able to enter into potential host cells