Question: How were viruses introduced into the human race?

Peter Elliott answered on 19 Jun 2014:

The simple answer is pure chance.

A virus is not a concious organism, it cannot think, feel or modify its behaviour. Some viruses have been part of the human race since its origin, a good example of this is the herpes virus. It is thought that our ancestors first got the virus from chimpanzees over 6 million years ago. Back then our similarity to chimpanzees would have been closer then today so the virus would have found it relatively simple to cross into a new host.

Other viruses then started to appear when we began to domesticate animals. Back then animals often shared very close quarters to humans and this gave mutating viruses a chance to cross from one host to another. This would have not been a quick process but it only needed one chance mutation to allow the virus to transfer. This process is still happening today as you may have come across bird flu and swine flu, two viruses that have, in a few cases, passed from other animals into humans and caused infection and death.