Every year, outbreaks of illness caused by E. coli make headlines. Here are the basics about the bacterium and its dangers, based on information from the U.S. Food and Drug Administration and the Centers for Disease Control and Prevention.

What is Escherichia coli?

E. coli is a large group of mostly harmless microbes that live in the guts of people and animals. However, some disease-causing strains can be life-threatening. The most worrisome is Shiga toxin-producing E. coli, which causes an estimated 265,000 infections a year in the United States. Harmful strains may be carried by some wildlife, livestock, and humans.

How is disease-causing E. coli spread?

Infections start when you swallow a tiny, probably invisible amount of human or animal feces. This contamination can be spread in several ways. Animal waste may come into contact with food or water. Food handlers may not wash their hands properly after going to the bathroom. Pets and petting zoos also can be sources of dangerous E. coli.

What are the symptoms of serious infection and how soon do they appear?

It usually takes three to four days after ingesting the bacteria to feel sick. Generally, the symptoms include severe stomach cramps, diarrhea, fever, nausea, and vomiting. The most virulent infections can cause bloody diarrhea, a type of kidney failure called hemolytic uremic syndrome, high blood pressure, neurologic problems, and even death in rare cases.

Who is at risk of infection?

While anyone can become infected, children under age 5, adults over age 65, and those with weakened immune systems are most likely to become seriously ill.

What foods are linked to E. coli outbreaks?

Raw or undercooked meat products used to be a major source of emergencies and food recalls. But even flour and cereal have been linked to outbreaks. In recent years, leafy greens, salad mixes, sprouts, and vegetables have become common sources of contamination.

Why are leafy greens such as romaine lettuce so vulnerable?

The popularity of convenience salad mixes — many containing romaine — and the fact that lettuce cannot be processed in a way that would kill the bacteria are fundamental factors. Leafy vegetables were responsible for 22% of food-borne illnesses between 1998 and 2008, according to one federal analysis. Women, who tend to eat more salads, are more affected by the outbreaks.

How are serious infections diagnosed?

Diagnosis involves laboratory testing of a stool sample. The CDC advises seeing a doctor if “you have diarrhea that lasts for more than three days, or is accompanied by high fever, blood in the stool, or so much vomiting” that you become dehydrated.

What is the treatment?

Supportive therapy, including plenty of fluids and anti-diarrheal medicines, is important. Antibiotics are not helpful and may increase the risk of kidney failure.

How can serious infections be prevented?

Wash your hands thoroughly after using the bathroom or changing diapers, after contact with animals, and before preparing or eating food. Cook meats thoroughly, and wash counters, cutting boards, and utensils after they touch raw meat. Avoid raw milk and unpasteurized dairy products. Avoid swallowing water in lakes, ponds, and pools.

Consumers can also submit a voluntary report or complaint to the FDA’s Center for Food Safety and Applied Nutrition.