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The transmissions of viruses through interspecies interaction have been an important source for new human viral diseases. This sort of "host switching" has allowed viruses to adapt from a host to host transmission. An important example of host switching is the viral transmission of HIV/AIDS. This cross host transfer between humans and primates could have been prevented through changes in demographic, cellular, and limited exposure. Even simple changes in social and demographic factors can be significant preventable measures to avoid viral transmission. Viral transfers must obstacle through several barriers to survive cross host transmissions; some including: environmental, demographic, genetic, and molecular. Some viral infections have even crossed between hosts that are closely related species. Mechanisms involved in the emergence of viruses have come from their evolution in quickly adapting to a host with different molecular properties, (i.e. DNA vs. RNA viruses)
Different steps are involved with the emergence include the: exposure, infection, spread, and adaptation. The exposure level demands specific requirements to allow for host switching. For example, there is only a limited opportunity for the exposure that is required for sufficient viral transmission. These opportunities depend on things such as environmental interactions, specific transmission encounters, and behavioral contact rates. Interaction within the same environment can increase the likelihood of contact between host and virus. Deforestation and agricultural expansion could cause species with zoonotic diseases to travel into areas which are already inhabited by humans. Social and demographic changes can promote emergence when certain behaviors are changed. The infection can only occur if the virus is compatible with host, within range of transmission, and if the virus is able to void all of the biological barriers. The compatibility of host to virus can depend and be limited to entry, vehicle, receptor binding, and genomic replication/expression. Because viruses are host specific, the barrier of different hosts require viruses to change between different proteins. Sometimes spillover occurs. Spillover is when an epidemic infection is transmitted from one species to a closely related species or distantly related species. This spillover occurred in the HIV virus from chimpanzee to humans and is also responsible for the transmission between the SARS CoV virus of bats to humans. Spreading of the virus occurs within the molecular level through tissue specificity binding and receptor binding. Viruses lacking the needed enzymes to bind to glycans to stop inactivation would need to adapt to the host environment to avoid destruction. When viruses switch hosts, the binding of the receptors from the prior host can be different from the new, not allowing for viruses to bind effectively to the same receptor cells. The adaptation may require multiple changes in the virus that could ultimately lead to a completely different binding site for the virus. The avian/mammalian flu virus was adapted to changing from the binding sites of the intestinal tracts of waterfowl to binding to the respiratory tract of humans and other mammals. Adaptation occurs in viruses when recombination and assortment of genes occur to match a specific host, but this process could take months to years to successfully complete. Some evidence that suggests that adaptations could occur is seen in the avian flu virus. Being that the virus only affected the avian species, its pathogenicity suggest that maybe it could have been trying to fully adapt to the human environment. Further evidence of viral adaptations could be seen in the Nipah virus collected at the beginning of an outbreak as compared to ones collected at the end. They were comparably different from each other and could have possibly been the result of one or more mutations as to adapt to the human body.
Preventative measures can only be used to a certain point with its considerable potential to host switching. Public health measures along with a combination of vaccinations have been quite successful in preventing the spread of viral infections. Our inability to predict the evolution and susceptibility of viruses leaves us to generically plan for the worst through early detection, classical quarantine, and vaccination strategies. By detecting the viruses early, contact between the reservoir and hosts can be drastically reduced, that could lead to the extermination or prevention of outbreak. Unfortunately, new viruses are almost impossible to prevent when there a certain amount of people are infected beyond threshold, therefore, by quarantine and a combination of early detection, this number can be reduced. Vaccinations are only work to a certain degree as it is mostly used to control the virus rather than eliminate the virus. The production and control of vaccines are only substantially made at a certain rate, and would not supply enough to control new emerging viral diseases. The virus' ability to switch hosts will result in more future human zoonotic diseases, but the prevention of transmission can benefit public health if educated as necessary.
Zoonotic diseases can be easily avoided by simply being careful around animals. The use of vaccines and preventative measures have led to the termination of some viruses (i.e. smallpox), but even with all those methods, viruses cannot be predicted because of their ever changing adaptations. One way of avoiding a tragedy as this could be avoiding interactions with other species. By avoiding the species all together, the risk of transmittance becomes closer to zero. Another way of effective preventative measures is by taking vaccines to counteract any chance of transmittance that could later happen in the future. By taking the simple preventive measures to stop the first step of exposure, mostly nothing would get past the first stage for infection and spread.