Escherichia coli refers to a diverse family of hundreds of bacteria, many of which are permanent residents of human intestines, serving a beneficial purpose in digestion. The potentially deadly strain that has received recent publicity was first described in 1982, and is known as E. coli O157:H7. This strain of the bacteria produces a substance known as Vero-cytotoxin, which can cause severe illness, characterized by bloody diarrhea and occasional kidney failure in children and the elderly. Symptoms normally appear between three to six days after ingestion of the bacteria. Most illness associated with E. coli has been traced to eating undercooked, contaminated ground beef, although it can also be transmitted via person-to-person contact, by eating raw milk, contaminated vegetables or apple cider, and by swimming in or drinking sewage-contaminated water. The organism lives in the intestine of healthy cattle, and meat can become contaminated during slaughter. Because grinding mixes the bacteria into the product, ground meats represent a greater threat than do whole cuts. Contaminated meat looks and smells normal. Raw milk can be contaminated from bacteria present on a cow's udder. It appears that even small amounts of this organism can cause severe illness.