Guess what? Even when we know it happens, it can still surprise. Case in point, the current dominant strain of influenza that is causing people to use up all their sick days at work this winter happens to have be a "mutant" strain that is resistant to one of the newest and best antiviral drugs on the market.
Not only is the H1N1 strain resistant, but the very mutation that causes that resistance has been identified. It's a spontaneous mutation (apparently not caused by selective pressure from the use of antiviral drugs), but the sudden increase and overwhelming replacement this season in the flu genome is tremendous. The mutation was identified a few years ago, but was contained to a small percentage of the total cases. In fact, last year the resistant variety was less than 11% of the total flu cases. This year -- 99%.
Now, this isn't the dreaded pandemic of "bird flu" -- but it does show the incredible power and effect of mutation plus natural selection, i.e., evolution. But one of the biggest problems is the lack of a rapid differential identity test between strains. Not even in the biggest hospitals can the doctors tell which strain a sick person has until it's too late to start on the antiviral treatment (it has to be started within 48 hours of infection to be effective).
Lucky for us humans, the flu isn't capable of horizontal gene transfer -- er. . . wait a minute, yes it is. Hmmm. Better keep that box of tissues handy, the drugstore might not be able to help you in a few years!