Signs and symptoms of E. coli O157:H7 infections typically begin three or four days after exposure to the bacteria, though you may become ill as soon as one day afterward to more than a week later. Signs and symptoms include:

• Diarrhea, which may range from mild and watery to severe and bloody
• Abdominal cramping, pain or tenderness
• Nausea and vomiting, in some people

When to see a doctor
Contact your doctor if:

• You become ill after eating fresh produce or undercooked ground beef
• Your diarrhea is persistent or severe
• Your diarrhea is bloody

Your doctor may test a stool sample for E. coli bacteria. Most E. coli infections — even those caused by O157:H7 — clear up without treatment.

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Among the many strains of E. coli, only a few trigger diarrhea. One group of E. coli — which includes O157:H7 — produces a powerful toxin that damages the lining of the small intestine, which can cause bloody diarrhea.

You develop an E. coli infection when you accidentally ingest the bacteria. Potential sources of exposure include contaminated food or water, and person-to-person contact.

Contaminated food

• Ground beef. When cattle are slaughtered and processed, E. coli bacteria in their intestines can get on the meat. Ground beef combines meat from many different animals, increasing the risk of contamination. Contaminated ground beef often has bacteria spread throughout the mixture — not just on the surfaces (such as on a steak), where cooking is more likely to destroy bacteria.
• Unpasteurized milk. E. coli bacteria on a cow's udder or on milking equipment can get into raw milk.
• Fresh produce. Runoff from cattle farms can contaminate fields where fresh produce is grown. Vegetables such as spinach and lettuce are particularly vulnerable to this type of contamination.

Contaminated water
Human and animal feces may pollute ground and surface water, including streams, rivers, lakes and water used to irrigate crops. Drinking or inadvertently swallowing untreated water from lakes and streams can cause E. coli infection.

Although public water systems use chlorine, ultraviolet light or ozone to kill E. coli, some outbreaks have been linked to contaminated municipal water supplies. Private wells are a greater cause for concern. Some people have been infected after swimming in pools or lakes contaminated with feces.

Personal contact
E. coli bacteria can easily travel from person to person, especially when infected adults and children don't wash their hands properly. Family members of young children with E. coli infection are especially likely to acquire it themselves.

Restaurant workers who don't wash their hands after using the bathroom can transmit E. coli bacteria to food. Outbreaks have also occurred among children visiting petting zoos and in animal barns at county fairs.

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E. coli can affect anyone who is exposed to the bacteria. But some people are more likely to develop problems than others. Risk factors include:

• Age. Young children and older adults are at higher risk of experiencing illness caused by E. coli and more-serious complications from the infection.
• Weakened immune systems. People who have weakened immune systems — from AIDS or drugs to treat cancer or to prevent the rejection of organ transplants — are more likely to become ill from ingesting E. coli.
• Eating certain types of food. Risky foods include undercooked hamburger; unpasteurized milk, apple juice or cider; and soft cheeses made from raw milk.
• Stomach-reduction surgery. People who've had surgery to reduce the size of their stomachs are more likely to develop symptoms from E. coli, possibly because they have less stomach acid to kill the bacteria.

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Most healthy adults recover from E. coli illness within a week. But some people — particularly young children and older adults — may develop a life-threatening form of kidney failure called hemolytic uremic syndrome (HUS).

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Most people don't seek medical attention for E. coli infections. But if you do go to the doctor, he or she will need to know:

• When the illness began
• The frequency of the vomiting or diarrhea
• Whether the vomit or diarrhea contains bile, mucus or blood
• If you have a fever or abdominal pain
• If you've recently traveled outside the country

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To diagnose illness caused by E. coli infection, your doctor will send a sample of your stool to a laboratory to test for the presence of E. coli bacteria. The bacteria may be cultured to confirm the diagnosis and identify specific toxins, such as those produced by E. coli O157:H7.

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For illness caused by E. coli O157:H7, no current treatments can cure the infection, relieve symptoms or prevent complications. For most people, the best option is to rest and drink plenty of fluids to help with dehydration and fatigue. Avoid taking an anti-diarrheal medication — this slows your digestive system down, preventing your body from getting rid of the toxins.

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Follow these tips to prevent dehydration and reduce symptoms while you recover:

• Clear liquids. Drink plenty of clear liquids, including water, clear sodas and broths, gelatin, and juices. Avoid apple and pear juices, caffeine and alcohol.
• Add foods gradually. When you start feeling better, stick to low-fiber foods at first. Try soda crackers, toast, eggs or rice.
• Avoid certain foods. Dairy products, fatty foods, high-fiber foods or highly seasoned foods can make symptoms worse.

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No vaccine or medication can protect you from E. coli-based illness, though researchers are investigating potential vaccines. To reduce your chance of being exposed to E. coli, avoid risky foods and avoid cross-contamination.

Risky foods

• Avoid pink hamburger. Hamburgers should be well-done. Meat, especially if grilled, is likely to brown before it's completely cooked, so use a meat thermometer to ensure that meat is heated to at least 160 F (71 C) at its thickest point. If you don't have a thermometer, cook ground meat until no pink shows in the center.
• Drink pasteurized milk, juice and cider. Any boxed or bottled juice kept at room temperature is likely to be pasteurized, even if the label doesn't say so.
• Wash raw produce thoroughly. Although washing produce won't necessarily get rid of all E. coli — especially in leafy greens, which provide many spots for the bacteria to attach themselves to — careful rinsing can remove dirt and reduce the amount of bacteria that may be clinging to the produce.

Avoid cross-contamination

• Wash utensils. Use hot soapy water on knives, countertops and cutting boards before and after they come into contact with fresh produce or raw meat.
• Keep raw foods separate. This includes using separate cutting boards for raw meat and foods such as vegetables and fruits. Never put cooked hamburgers on the same plate you used for raw patties.
• Wash your hands. Wash your hands after preparing or eating food, using the bathroom or changing diapers. Make sure that children also wash their hands before eating, after using the bathroom and after contact with animals.

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