They also suppress the growth of harmful bacteria in our intestines. E. coli—short for Escherichia coli—are known as "enteric" bacteria. "Enteric" comes from the Greek word for "intestine."

A symbiotic relationship means that two organisms that aren't similar live together in a way that helps both. People provide a good place for them to hang out (the intestines). They help you live by making vitamins needed for your health and promoting a healthy intestinal tract. Note that these helpful bacteria live only in areas of the body that are exposed to air: the intestines and respiratory tract. In a healthy person, E. coli is never found in the bloodstream or body tissues.

There are hundreds of strains of E. coli, and most of them are harmless. But some strains can cause illness. For example, the strain E. coli 0157:H7, often shortened to just E. coli 0157, is entirely different from the strains that typically inhabit our intestines. E. coli 0157 produces a toxin that damages the walls of the intestine causing hemorrhaging, or loss of blood, in the intestines.

It is always found around animals—typically, cattle—which is why children who have visited petting zoos may become ill if they don't wash their hands after petting the animals and before eating. It can be found in contaminated, undercooked meat. E. coli can also live on a cow's udders, so it can be found in unpasteurized milk products. It can easily spread to other food sources through contaminated water.

Cases that involve undercooked hamburger can be blamed on the strain of E. coli found in the intestines of cattle. It can contaminate the beef during slaughter. Hamburger meat is more susceptible to contamination because the bacteria are ground into the meat's interior, where temperatures don't get as high during cooking. Other contaminated foods include unpasteurized milk, salami, lettuce, alfalfa and radish sprouts, game meat, and cheese curds. The bacteria also can be spread from one person to another through families and child-care centers that don't practice good hygiene. Another mode of transmission is by swimming in or drinking water that has been contaminated with fecal material. This can include swimming pools, wading pools, unchlorinated water, and groundwater. It is thought that it takes only a small number of bacteria to cause an infection, maybe as few as 10 organisms.

The bacteria produce a toxin (Shiga toxin or verotoxin) that damages the cells that line the walls of the intestine. The illness is called hemorrhagic colitis, which causes severe illness involving a loss of water, salts, and blood. A serious complication of this infection is hemolytic uremic syndrome, a rare condition affecting mostly very young children and elderly adults. In HUS, the toxin enters the bloodstream and destroys red blood cells and platelets and damages blood vessel walls. The damaged cells and other debris from the destruction can block small blood vessels in the kidney or damage the kidney itself. The kidneys may stop working. In severe cases, the kidneys suffer permanent damage.

Hemolytic uremic syndrome, a complication of the infection, is fatal to about half the older adults who develop it, and a small percentage of young children.

Symptoms include severe abdominal cramping and bloody diarrhea. Other possible symptoms include nausea, vomiting, and low-grade fever.

Supportive therapy is the mainstay of treatment and includes fluids and electrolytes. In severe cases, a person may need blood transfusions and dialysis. There is no evidence that antibiotics can help treat this illness, and they may make it worse, experts say. The disease usually resolves itself after 5 to 10 days. But children may need a longer hospital stay. Treatment for hemolytic uremic syndrome may include blood transfusions, medicines, fluids, and possibly dialysis. The condition may cause death if untreated.

When cooking hamburger, make sure the internal temperature has reached 160°F (71°C) to ensure that all bacteria have been killed. Contaminated meat looks and smells normal; only a small amount of E. coli 0157 bacteria need be present to cause illness. Never place cooked meat on a plate that held raw meat unless the plate has been thoroughly washed. The same goes for utensils. In general, be sure to wash your hands thoroughly after using the toilet and before eating. It's also important to avoid raw milk and unpasteurized dairy products and fruit juices. Also, avoid swallowing water in lakes, ponds, streams, and swimming pools.