Humans have been getting infected by viruses since the dawn of human evolution. For thousands of years, small pox, influenza, polio, herpes and human papiloma virus have been a huge part of human existence. But it has only been in the last one hundred years that there has been a name and identifying characteristics of viral infections. Until then, these diseases carried superstitions and rituals for how to avoid getting infected. Until Jenner, the physician who administered the first smallpox vaccine, noticed that milkmaids did not get small pox when they had been milking cows infected with cow pox, unseen infectious agents making people sick was something that was thought only the devil or other evil forces would do to bad or unworthy people.
The perception that viral infections only apply in humans is not correct. Viruses have evolved along with their host of choice to continue to infect and replicate their genes. Mammals, birds, reptiles, fish, plants and fungi are all known to have their own types of viral infections. Some of the most damaging and detrimental viral infections do not occur in humans, but affect livestock and food crops.
A virus is one of the most infectious types of pathogens that an organism can acquire. Strangely enough, a virus is not considered to be a living organism, as it does not need to metabolize carbohydrates or proteins to remain alive. In fact, a virus, (or known singularly as a viroid,) does not possess a body with any kind of organelles in order to process or undergo metabolism. To put things simply, a viroid is nothing more than genetic material wrapped in protein. So how does a virus 'live?' When a viroid infects a host cell, it inserts specific genes into the host cell’s nucleus and effectively hijacks and reprograms the cell to start replicating more of the virus. This is where it is crucial that the virus infects the correct host, as it needs specific cells to carry out the replication process.
Humans are more sensitive to certain viral infections due to the fact that these viruses are specific to the human species. If humans did not exist, there would probably not be any of the most commonly known viral infections that have been associated with human health throughout history.
Humans rarely get viral infections that are specific to other animals, but unfortunately, this is not always the case. It is true that humans can get influenza, originally from a bird, as this particular strain of influenza is genetically able to cross into two different species due to mutant genes. This is also the theory for the origins of HIV and the Ebola virus, as both of these viruses have been seen in primate populations living near humans.
Human viruses play a very important part in evolution both for humans and the viruses themselves. Unlocking the mysteries of viral infections has been a crucial part in the development and production of anti-viral medications for patients with HIV and AIDS. The close connection that viruses have with humans is detrimental, but it can also be seen as extremely useful.