In common usage, an antibiotic is a drug that kills certain kinds of bacteria, but which is generally harmless to the host and is used to treat infection. Strictly speaking, the term can also apply to substances that affect prions, viruses, fungi, worms or any other intracellular or extracellular parasite, but the antibacterial kind are the most common.
The first antibiotic to be discovered was penicillin. Its discoverer, Alexander Fleming, had been culturing bacteria on an agar plate with an accidental fungal contamination, and noticed that the culture medium around the mold was free of bacteria. He had previously worked on the antibacterial properties of lysozyme, and so was predisposed to make the correct interpretation of what he saw: that the mold was secreting something that stopped bacterial growth. Though he was unable to produce the pure material (the beta-lactam ring in the penicillin molecule making it unstable under the purification methods he tried), he reported it in the scientific literature. Since the mold was of the genus Penicillium, he named this compound penicillin. With the increased need for treating wound infections in World War II, resources were poured into investigating and purifying this compound, and antibiotics came into widespread use.
The discovery of antibiotics, along with anesthesia and the adoption of hygenic practices by physicians (for example, washing hands and using sterilized instruments) revolutionized medicine. They are often called "magic bullets": drugs which target bugs without greatly harming the host.
There are several classes of antibacterial antibiotics in common use today. These are:
Common forms of antibiotic misuse include taking an antibiotic for an inappropriate condition, in particular the use of antibiotics for viral infections; and not taking the entire course of the antibiotic, usually because the patient feels better before the infection is cured.
There is debate over the appropriateness of including antibiotics in the diet of healthy farm animals. Opponents of this practice point out that it leads to antibiotic resistance, including in bacteria that infect humans. The practice continues in many places, however, because feeding livestock antibiotics promotes weight gain, and thus makes economic sense for the individual farm or ranch.
One side effect of misusing antibiotics is the development of antibiotic resistance by bacteria. By 1984 half the people with active tuberculosis in the United States had a strain that resisted at least one antibiotic. Between 1985 and 1991 tuberculosis increased 12 per cent in the US and 300 per cent in Africa where HIV and TB are often found together.