Antimicrobial agent, any of a large variety of chemical compounds and physical agents that are used to destroy microorganisms or to prevent their development. The production and use of the antibiotic penicillin in the early 1940s became the basis for the era of modern antimicrobial therapy. Streptomycin was discovered in 1944, and since then many other antibiotics and other types of antimicrobials have been found and put into use. A major discovery following the introduction of these agents into medicine was the finding that their basic structure could be modified chemically to improve their characteristics. Thus, antimicrobial agents that are used in the treatment of disease include synthetic chemicals as well as chemical substances or metabolic products made by microorganisms and chemical substances derived from plants. Although the term antibiotic is popularly used to refer to drugs that combat microbes, strictly speaking, it refers only to drugs that kill or inhibit bacteria.