The Centers for Disease Control says E. coli bacteria are a large and diverse group of bacteria.

Although most strains of E. coli are harmless, others can make you sick.

Some kinds of E. coli can cause diarrhea, while others cause urinary tract infections, respiratory illness and pneumonia, and other illnesses.

Still other kinds of E. coli are used as markers for water contamination—so you might hear about E. coli being found in drinking water, which are not themselves harmful, but indicate the water is contaminated.

Here are some questions and answers from the CDC:

What is E. coli?

E. coli is a common kind of bacteria that lives in the intestines of animals and people. There are many strains of E. coli. Most are harmless. However, one dangerous strain is called E. coli O157:H7. It produces a powerful poison. You can become very sick if it gets into your food or water.

In 1999 it was estimated that about 73,000 people in the U.S. got sick each year from E. coli. About 60 died. It’s believed that the number of illnesses and deaths has been dropping since then.

How is E. coli O157:H7 spread?

Outbreaks often are caused by food that has gotten the bacteria, E coli, in it. Bacteria can get accidentally mixed into ground beef before packaging. Eating undercooked meat can spread the bacteria, even though the meat looks and smells normal. E. coli can also live on cows’ udders. It may get into milk that is not pasteurized.

Raw vegetables, sprouts, and fruits that have been grown or washed in dirty water can carry E. coli O157:H7. It can get into drinking water, lakes, or swimming pools that have sewage in them. It is also spread by people who have not washed their hands after going to the toilet.

E. coli can be spread to playmates by toddlers who are not toilet trained or by adults who do not wash their hands carefully after changing diapers. Children can pass the bacteria in their stool to another person for 2 weeks after they have gotten well from an E. coli O157:H7 illness. Older children and adults rarely carry the bacteria without symptoms.

What are the signs of E. coli O157:H7 sickness?

Bloody diarrhea and stomach pain are the most common signs of E. coli O157:H7 sickness. People usually do not have a fever, or may have only a slight fever.

Some people, especially children under 5 and the elderly, can become very sick from E. coli O157:H7. The infection damages their red blood cells and their kidneys. This only happens to about 1 out of 50 people, but it is very serious. Without hospital care, they can die. See a doctor right away if you think you may have gotten sick from E. coli O157:H7.

How will my doctor know if E. coli O157:H7 made me sick?

Your doctor will test to see if your sickness was caused by E. coli by sending a stool sample to a lab. The lab will test for the bacteria.

Anyone who suddenly has diarrhea with blood in it should call or see a doctor.

How is it treated?

Your doctor will tell you what is best. Taking medicine on your own may not help you get better, and it could make things worse. Do not take antibiotics or diarrhea medicine like Imodium® unless your doctor tells you to.

Will E. coli O157:H7 infection cause problems for me later?
People who have only diarrhea and stomach ache usually get completely well in 5-10 days. They do not have problems later.

For those people who get very sick and have kidney failure, about 1 out of 3 may have kidney problems later. In rare cases, people have other problems like high blood pressure, blindness, or are paralyzed. Talk to your doctor if you have questions about this.