Any substance poisonous to an organism; often restricted to poison s produced by living organisms.
In addition to those from such microorganisms as bacteria (see bacterial diseases ), dinoflagellate s, and algae , there are toxins in fungi ( mycotoxin s; see aflatoxin ; mushroom poisoning ), higher plants (phytotoxins), and animals (zootoxins, or venom s). The plants include nightshade (see nightshade family ), poison hemlock , foxglove , mistletoe , and poison ivy . Many plant toxins (e.g., pyrethrins, nicotine , rotenone) apparently protect their producers against certain animals (especially insects) or fungi. Similar defensive secretions in animals may be widely distributed or concentrated in certain tissues, often with some sort of delivery system (e.g., spines, fangs). Animals such as spiders and snakes use venoms to catch prey and often for defense. Many normally edible fishes and shellfishes become poisonous after feeding on toxic plants or algae. See also antidote ; food poisoning .