DRUG RESISTANCE: Bacteria are highly adaptive, and over time they naturally develop resistance, which protects them from incoming germs (and antibiotics) and makes them more difficult to kill. If someone has strep throat, for example, repeated exposure to penicillin and amoxicillin can result in a throat full of bacteria, which shields strep germs from the older drugs. The surviving bacteria then reproduce and become more dominant. Sometimes parents discontinue antibiotic medication prematurely when they or their children begin to feel better, so the strep germs aren't entirely killed off, leading to much more severe infections requiring the use of even stronger drugs later on. This can also happen with many other infections inside the body and on the skin.
HOW ANTIBIOTICS WORK: Infections are caused by single-celled organisms called bacteria, which can sometimes evade the body's immune system and begin reproducing. Antibiotics kill those harmful bacteria in various ways, such as preventing a bacterium from turning glucose into energy, or preventing it from construct a cell wall. The bacteria die instead of reproducing. Antibiotics are like selective poisons, because they target bacteria and not the body's own cells. They are not effective against viruses, however. Unlike bacteria, a virus isn't a living, reproducing lifeform, just a piece of DNA or RNA. A virus injects its DNA into a living cell and the cell itself reproduces more of the viral DNA. There is nothing to 'kill', so antibiotics don't work on viruses.