An antimicrobial is a drug used to treat a microbial infection; a general term that refers to a group of drugs that includes antibiotics, antifungals, antiprotozoals, and antivirals. Bacteria, fungi, and viruses are responsible for almost all of the infectious diseases. Many disorders formerly thought to be caused by other factors, like stress, are now known to be caused by bacteria. For example, many ulcers are caused by the bacteria Helicobacter pylori, and not by stress, as originally believed. Historically, antimicrobials began with the observations of Pasteur and Joubert, who discovered that something made by one type of bacteria could prevent the growth of another. Technically, antibiotics are only those compounds produced by one microorganism that kill, or prevent the growth, of another microorganism. However, in today’s common usage, the term “antibiotic” is used to refer to almost any drug that stops a bacterial infection. Antimicrobials include not just antibiotics, but synthetically formed compounds as well.