The biologists Valerie Curtis and Adam Biran identify the reason. It can't be a coincidence, they note, that the most disgusting substances are also the most dangerous vectors for disease. Feces is a route of transmission for the viruses, bacteria, and protozoans that cause at least 20 intestinal diseases, as well as ascariasis, hepatitis A and E, polio, ameobiasis, hookworm, pinworm, whipworm, cholera, and tetanus. Blood, vomit, mucus, pus, and sexual fluids are also good vehicles for pathogens to get from one body into another. Although the strongest component of the disgust reaction is a desire not to eat or touch the offending substance, it's also disgusting to think about effluvia, together with the body parts and activities that excrete them. And, because of the involuntariness of speech perception, it's unpleasant to hear the words for them.