Antibiotics kill bacteria, but are generally harmless to humans.
This is due to our dramatically different cellular structures. Bacteria have cell walls
that protect them from the environment (e.g your digestive juices). Antibiotics play havoc
with cell wall synthesis or function. Thus, while the drugs do precious little to our
cells, they do cause bacteria to lose their armor, thus exposing them to the harsh environs.
Bacteria cause a great many diseases, including tetanus, syphilis, strep throat and leprosy. The discovery of antibiotics thus revolutionized the medical field. They are useless against viruses though, so they're still useless against the common cold and the flu!
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