In biology, poisons are substances that can cause death, injury or harm to organs, usually by chemical reactions or other activity on the molecular scales, when an organism is exposed to a sufficient quantity.The fields of medicine (particularly veterinary) and zoology often distinguish a poison from a toxin, and from a venom. Toxins are poisons produced by organisms in nature, and venoms are toxins injected by a bite or sting (this is exclusive to animals). [Source: Wikipedia ]