Do you know definitively where the common cold came from?
How about the new strains of influenza that seem to come out every year? One of those strains back in 1918 managed to kill about 20 million people, twice the death toll of WW-I.
A not so funny thing about many viral strains is how rapidly they mutate. That's why it has proven so difficult to develop a HIV vaccine (or a cold vaccine, for that matter). Some scientists believe the HIV virus is a mutated form of a monkey virus that might've jumped to humans.
