Dido:
Are you saying that the same bacteria stays in your body, and changes? If that`s the case, the antibiotics are only keeping it under control, and it doesn`t entirely leave the body then?
Aye, that's exactly the case. It's usually caused by not taking the full course of antibiotics. You manage to kill enough of the bacteria to stop the symptoms, but don't kill them off completely. The bacteria that survive evolve and develop defenses, and not only come back with a vengance but go on to infect other people. Unfortunately the resultant "superbug" is hard to kill, so the individual who doesn't take the full course of medicine ends up harming both themselves and society at large.
...take aids for example.
AIDS (and the flu) is a virus. These mutate and evolve, too, but there are currently no antibiotics for them. They work in a different way by actually infecting the living cells in your body with RNA code (similar to DNA) and commandeering them for their own ends. This is different from bad bacteria, which are cells in their own right, and go around eating up anything around that they see as food Good bacteria are ok, as they only eat up stuff that is generally harmful to us, or break down stuff that we can't digest anyhow.
Aint science a wonderful thing