Venom is a type of poison, especially one secreted by an animal. Venom has evolved in a wide variety of animals, both predators and prey, and both vertebrates and invertebrates. Venoms kill through the action of at least four major classes of toxin, namely necrotoxins and cytotoxins, which kill cells; neurotoxins, which affect nervous systems; and myotoxins, which damage muscles. Biologically, venom is distinguished from other poisons in that unlike ingested poisons, venom is delivered in a bite, sting, or similar action. Venomous animals cause tens of thousands of human deaths per year. However, the toxins in many venoms have potential to treat a wide range of diseases.