We already know that bacteria adapts to antibiotics and that's a bit of a dangerous deal since antibiotics are the only thing we really have so far to fight bacteria, so if it adapts too much it will kill us all. In fact, I am 100% sure that is how we will either enter an apocalypse, zombie or otherwise.
Scientists recently decided to look into this further, and it turns out that bacteria will actually adapt to a new antibiotic within days, but remain dormant and wait for the antibiotics to run their course before activating again. Because they are survival machines.
To me this means we're even more screwed than we realized, but the scientists seem to think this could be used to develop better ways of fighting bacteria. Let's hope.