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

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

When to see a doctor

Contact your doctor if your diarrhea is persistent, severe or bloody.

Only a few strains of E. coli trigger diarrhea. The E. coli O157:H7 strain belongs to a group of E. coli that produces a powerful toxin that damages the lining of the small intestine. This can cause bloody diarrhea. You develop an E. coli infection when you ingest this strain of bacteria.

Unlike many other disease-causing bacteria, E. coli can cause an infection even if you ingest only small amounts. Because of this, you can be sickened by E. coli from eating a slightly undercooked hamburger or from swallowing a mouthful of contaminated pool water.

Potential sources of exposure include contaminated food or water and person-to-person contact.

Contaminated food

The most common way to get an E. coli infection is by eating contaminated food, such as:

• 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.
• 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. Certain vegetables, such as spinach and lettuce, are particularly vulnerable to this type of contamination.

Contaminated water

Human and animal stool may pollute ground and surface water, including streams, rivers, lakes and water used to irrigate crops. Although public water systems use chlorine, ultraviolet light or ozone to kill E. coli, some E. coli outbreaks have been linked to contaminated municipal water supplies.

Private water wells are a greater cause for concern because many don't have a way to disinfect water. Rural water supplies are the most likely to be contaminated. Some people also have been infected with E. coli after swimming in pools or lakes contaminated with stool.

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 get it themselves. Outbreaks have also occurred among children visiting petting zoos and in animal barns at county fairs.

E. coli can affect anyone who is exposed to the bacteria. But some people are more likely to develop problems than are 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 from drugs to treat cancer or prevent the rejection of organ transplants — are more likely to become ill from ingesting E. coli.
• Eating certain types of food. Riskier foods include undercooked hamburger; unpasteurized milk, apple juice or cider; and soft cheeses made from raw milk.
• Time of year. Though it's not clear why, the majority of E. coli infections in the U.S. occur from June through September.
• Decreased stomach acid levels. Stomach acid offers some protection against E. coli. If you take medications to reduce stomach acid, such as esomeprazole (Nexium), pantoprazole (Protonix), lansoprazole (Prevacid) and omeprazole (Prilosec), you may increase your risk of an E. coli infection.

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

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 swallowing water from lakes or pools, wash your hands often, avoid risky foods, and watch out for cross-contamination.

Risky foods

• Cook hamburgers until they're 160 F (71 C). Hamburgers should be well-done, with no pink showing. But color isn't a good guide to know if the meat is done cooking. Meat — especially if grilled — can brown before it's completely cooked. Use a meat thermometer to ensure that meat is heated to at least 160 F (71 C) at its thickest point.
• 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. Avoid any unpasteurized dairy products or juice.
• Wash raw produce thoroughly. Washing produce may not get rid of all E. coli — especially in leafy greens, which provide many places 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.

To diagnose illness caused by E. coli infection, your doctor sends 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.

For illness caused by E. coli, no current treatments can cure the infection, relieve symptoms or prevent complications. For most people, treatment includes:

• Rest
• Fluids to help prevent dehydration and fatigue

Avoid taking an anti-diarrheal medication — this slows your digestive system down, preventing your body from getting rid of the toxins. Antibiotics generally aren't recommended because they can increase the risk of serious complications and they don't appear to help treat the infection.

If you have a serious E. coli infection that has caused a life-threatening form of kidney failure (hemolytic uremic syndrome), you'll be hospitalized. Treatment includes IV fluids, blood transfusions and kidney dialysis.

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

• Drink clear liquids. Drink plenty of clear liquids, including water, clear sodas and broths, gelatin, and juices. Avoid apple and pear juices, caffeine, and alcohol.
• Avoid certain foods. Dairy products, fatty foods, high-fiber foods or highly seasoned foods can make symptoms worse.
• Eat meals. When you start feeling better, you can return to your normal diet.

Most people don't seek medical attention for E. coli infections. If your symptoms are particularly severe, you may want to visit your primary care doctor or seek immediate care.

Here's some information to help you get ready for your appointment and know what to expect from your doctor.

What you can do

• Write down any symptoms you're experiencing, including any that may seem unrelated to the reason for which you scheduled the appointment.
• Write down key personal information, including any recent life changes or international travel.
• Make a list of all medications, vitamins or supplements that you're taking.
• Ask a family member or friend to come with you, if possible. Sometimes it can be difficult to remember all of the information provided to you during an appointment. Someone who accompanies you may remember something that you missed or forgot.
• Write down questions to ask your doctor.

For an E. coli infection, some basic questions to ask your doctor include:

• What's the most likely cause of my symptoms?
• What kinds of tests do I need?
• What treatments are available, and which do you recommend?
• Will there be any lasting effects from this illness?
• How can I prevent this from happening again?
• Are there any brochures or other printed material that I can take home with me? What websites do you recommend visiting?

What to expect from your doctor

Your doctor may ask:

• When did your symptoms begin?
• How often are you having diarrhea?
• Are you vomiting? If so, how often?
• Does your vomit or diarrhea contain bile, mucus or blood?
• Have you had a fever? If so, how high?
• Are you also having abdominal cramps?
• Have you recently traveled outside the country?
• Does anyone else in your household have the same symptoms?

What you can do in the meantime

If you or your child has an E. coli infection, it may be tempting to use an anti-diarrheal medication, but don't. Diarrhea is one way the body rids itself of toxins. Preventing diarrhea slows that process down.

Take small sips of fluid as tolerated to try to stay hydrated.