Viruses have been impacting human society for thousands of years. Viruses such as small pox and polio have devastated populations for thousands of years and it wasn't until the invention of the vaccine that fully eradicated these diseases. (2) Polio and small pox are examples of what I would call "old viruses." There are now new, emerging viruses that are causing substantial financial costs and are major threats to the public health. (1) Moreover, these viruses are the results of "spillovers!" Spillovers are infections in alternative hosts. For humans, we are now serving as hosts for animal diseases! These diseases are called zoonotic diseases and are of major importance to human health. (1)
Viruses such as influenza, Ebola and HIV are examples of emerging diseases. The viruses are established animal viruses that have successfully switched hosts and now affect the human population. Influenza infects a wide variety of animals including, but not limited to, birds, pigs, horses, and poultry. (4) In fact, the H1N1 outbreak of April 2009 is composed of a combination of swine, avian, and human influenza viruses. (5) More than one spillover events had to occur. The emergence of H1N1 caused a huge uproar and people actually died from complications after infection.
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Ebola is a devastating disease in which humans actually have to be isolated if the disease is contracted. The Ebola virus natural hosts are unknown but rodents and arthropods are the usual hosts for the viral hemorrhagic fevers group, a group that Ebola is a member of. It is not a common zoonotic virus and the last known outbreak occurred in Zaire in 1995. Nevertheless, Ebola is highly contagious and little is known about its natural host, it also has the potential to reappear. (3)
There are three stages of viral disease emergence that will lead to complete host switching. The first is the initial stage of infection of a new host without any more transmission. This is referred to as spillover into "dead end" hosts. The second is spillovers that continue to cause regional chains of transmission in the human population before the epidemic disintegrates. Third, the epidemic has to be sustained in the host to host transmission and enter the new host population. (1)
In the new viral emergence, there are no specifics either. Viruses can be "generalists," which are viruses that affect different hosts or "specialists", which are viruses that affect one or a few closely related species. An example of this is SIV, a virus that affects primates and is similar to HIV in humans. (1) This is important because these viruses virtually have no limits and this is very unfortunate for the human population. This is seen with the case of HIV.
HIV/AIDS has been estimated to have emerged from primates into humans over 70 years ago. This is a recent example of host switching. Primates are our closest relative in the animal kingdom, which may have made it easier for host switching to occur. HIV has devastated millions of people worldwide and thus, became an epidemic. Another virus mentioned was smallpox, which created an epidemic for thousands of years, is thought to have originated in animals during prehistoric times. (1)
For a virus to establish an epidemic in a human population from an animal, the virus must go through genetic changes to infect a human. One genetic change that needs to occur is the ability to bind to a new receptor. Although we are eukaryotes, animal cells are quite different from human cells. Also, the viruses must go through mutations that allow each virus to escape the host intracellular defenses. Then, with the established infection, the virus will need to spread to a well populated area to maintain onward transmission. (1) With an epidemic, factors such as severity, number of people infected, and mode of transmission is very important. There are factors; however, that prevents epidemics in the human population. Zoonotic viral epidemics can be prevented for reasons such as environmental, demographic factors, and behavioral factors. For environmental, humans may have little or no contact with the viruses and its animal hosts. With demographical factors, humans may populate new areas or travel to new places. With humans, we have changed our behavioral patterns with animals. There are sexual practices with animals and also different farming practices. (1)
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Zoonotic viruses are very important to the human population viruses of animals. The major source of new human viral diseases are zoonotic viruses of animals. A large portion of viruses that affect wildlife and domestic animals are still unknown. It is important that research is funded to understand how viruses can cross species and establish a new host. Some factors that will help understand this phenomenon are understanding the host biological properties including cellular components, how the viruses gets from one host to another, and finally, where the animal host lives and travels. (1)
Zoonotic Viruses are very serious and certain preventative measures should be taken to contain these viruses. People should be careful when handling animals and being in contact with them. Also, people should not have sex with animals; this may decrease the spillover occurrence. Lastly, more information more information needs to be made available so more people can know about these disease and the animals that are carrying them.