A toxin is a poisonous substance produced by a living organism. Many microorganisms, animals, and plants produce chemical substances which are poisonous to some other organism; the toxin may be released continuously into the immediate environment or released only when danger is imminent. Examples include fungi which secrete substances that destroy bacteria (as antibiotics these are of great value to man) and poisonous spiders and snakes which deliver their toxins via fangs. In some organisms, the function of toxins is obscure, but in many others they play an important role in defense and in killing prey. The symptoms of many human infectious diseases (e.g., cholera, diphtheria, tetanus) are due to the release of toxins by the bacteria concerned. See also antitoxin.