Why is venom poisonous?

Each species of venomous spider or snake has a different mix of poisonous chemicals in its bite. Scientists know about two basic categories. One type of poison contains molecules that block the activity of nerves, which paralyzes the muscles. When this action stops the muscles that work the lungs, the prey stops breathing and dies. The other type of venom is made up of molecules that kill the prey by making tissues in the body begin to break down.