Antibiotics, also called antibacterials, are a type of antimicrobial drug used in the treatment and prevention of bacterial infections.

They may either kill or inhibit the growth of bacteria. A limited number of antibiotics also possess antiprotozoal activity. Antibiotics are not effective against viruses such as the common cold or influenza, and their inappropriate use allows the emergence of resistant organisms.

In 1928, Alexander Fleming identified penicillin, the first chemical compound with antibiotic properties. Fleming was working on a culture of disease-causing bacteria when he noticed the spores of a little green mold (Penicillium chrysogenum), in one of his culture plates. He observed that the presence of the mold killed or prevented the growth of the bacteria.