Bacteria and Viruses evolve. Viruses differently than Bacteria because they aren't living and only have either DNA or RNA (usually RNA). I guess you could say that Viruses can't evolve in the typical fashion that bacteria does because it is not a living organism.
Some viruses attack the RNA and DNA causing a mutation of the normal cells; sort of how camera works by changing the nucleic sequence of DNA.
If you don't great viruses and bacteria and then evolve them you will never know what to expect.
That is why the flu vaccine can never be 100%, because scientists are evolving influenza to create a new vaccine to accommodate the "evolved" or updated version of the virus. It is a guessing game with close to 200 known strands of influenza, each with their own RNA sequence and distinction.
What we really have to worry about is a virus or bacteria that rapidly evolves. I'm not talking about days or weeks, but minutes or seconds. As a Physician you would have to play Russian Roulette with treatment and would be treating the symptoms instead of the actually disease at that point because you wouldn't really know what you are treating or how to treat it.
