Antibiotic resistanc. Bacteria are able to adjust to exposure to the drugs that are designed to inhibit or kill them. It is a form of evolution. In a hospital, compared to other settings, the number of infections treated with antibiotics in very high. So the chances for these mututations is much higher. When patients become infected with bacteria from the hospital, it is often showing mulitple resistance.
From antibiotics! Bacteria divide and grow so fast that unless all bacteria are killed a cell or two may survive either by somehow being stronger or by having a transfer of genetic material with enzymes that break the antibiotic chemical in two. Only treating until you feel better may leave a few bacteria, those are more likely to start building resistance to the antibiotic.