Scientists have documented many, many species die offs throughout Earth's history. Not all of them correspond with any specific cataclysm like an asteroid impact. If fact, most of the die offs have happened independently of any other known major event. How many of these were related to viruses is any body's guess, but I am willing to bet that a majority of the die offs were.
Viruses can mutate easily. That is one of the reasons why flu viruses return every year with new capacity to attack human hosts. Cold viruses mutate regularly, too; and recent strains have proved more deadly as they can now easily lead to pneumonia.
With the massive increase and crowding of human populations (like in Asia), we are setting up the unsanitary grounds for a potential killer virus to evolve. When it does, it will blindside us by coming out of nowhere, just like Ebola or Marburg came from bush animals and SARS came from furry little animals in Asia.
We have been lucky so far in the past century. We have been able to contain those new viruses. Some species of animal out there has a virus, that, when appropriately mutated, will be able to break any means of containment and become a very effective eliminator of humanity. It is just a question of time, environmental conditions, and an opportune mutation.
We crowd chickens and turkeys in massive meat harvest farms, and in the process have found new ways to breed viruses in their populations. Bird Flu is a perfect example of that.
We crowd pigs together in massive pig meat harvesting farms, and in the process have found new ways to breed viruses in their populations. The new Swine Flu is a perfect example of that.
Now, we are cramping humans into massively overcrowded unsanitary population centers such as presently exist in southern Asia or the large cities of the poorer countries of the world. Viruses spread and mutate easily in conditions like that. What makes this condition even worse is that we now live in close proximity to those crowded turkey, chicken and pig farms that are providing protein for the masses.
Not good at all!