3. Freshwater Snail

One of the most unassuming killers on this list, the common freshwater snail is host to a horrifying surprise. These gastropods spread disease by releasing parasites into fresh water and can cause a condition known as schistosomiasis. This can lead to infected intestines, pain in the abdomen, and, if humans have been infected for a while, bladder cancer or kidney failure. Apparently, up to 200,000 people die from the illness each year.