Well this again is a fair point, but what about the mass extinctions(75% of all species) that are believed to be caused by a virus?
This statement requires a citation. Because there are very few viruses that can jump species easily, and usually are restricted to affecting only closely related species. One virus is not adaptable enough to take out 75% of all species. Even if you are saying that 75% of all extinctions in time have been caused by (different each time) viruses, I'd like to see a citation for that belief.
In 1993, Cesare Emiliani tried to make a case for viral extinctions, two papers I could find ("Extinction and Viruses" Biosystems vol 31 and "Viral Extinctions in Deep-Sea Species" Nature vol 366) Neither has made a very large impression upon the scientific community as the above papers have 5 and 6 citations, respectively (and that 6 includes 1 paper refuting Emiliani's idea). 11 Citations in 13 years is incredibly poor; all indications are that most active researchers do not agree.
The refutation by Buckwold ("Viral-induced Extrinctions Unlikely" Nature vol 368, 1994) is pretty simple: Viruses are obligate parasites, and as such their survival depends upon the host. If a virus were to be so overwhelming to the host as to kill the host, the virus dies too. A virus would have to be so virulent, and so able to mutate as to jump host species repeatedly if it caused extinctions. In Buckwold's words "A virulent virus can only eliminate a host population if the virus can also live in another host which it does not kill. In the absence of an alternative host population for these viruses, it is improbable that lethal mutant viruses arose repeatedly during evolution and completely eliminated their hosts."
If you have a different source for your virus extinction idea, I'd really appreciate it if you posted it.
and p.s. to both Xebadir and Lynden, when you post such long and detailed posts, it would really, really nice if you guys could cite some sources for these ideas. Even if it is just webpages, at least then everyone could look at the originals from where you formed your opinions and make an opinion of their own as well as critique the sources.