You can't see it, smell it or taste it. But food or water tainted with E. coli O157:H7 bacteria can leave you fighting for your life, especially if your immune system is compromised or you're very young or very old.

Seven people died and more than 2,300 others fell ill in Walkerton, Ont., in May 2000, in Canada's worst-ever E. coli outbreak after the bacteria got into the town's water supply. The source of the contamination was manure spread on a farmer's field near one of the town's wells.

While the Walkerton case was unusual, health authorities across the country normally deal with a few thousand cases of E. coli illness a year. In the U.S., it's estimated that 73,000 people are sickened by the bacteria every year and 61 people die. In March 2011, several people in the U.S. were being investigated for E. coli linked to in-shell hazelnuts and in the same month the U.S. Centers of Disease Control and Prevention also investigated a link to one kind of bologna. Two Canadians fell ill with a strain of E. coli that matched the genetic fingerprint of the strain responsible for the hazelnut illness in the United States.

In 2006, tainted spinach led to the deaths of three people and sickened 204 people, including one Canadian woman. In a separate outbreak, Ontario health authorities in October 2008 investigated an E. coli outbreak linked to a Harvey's restaurant in North Bay.

In July 2009, President's Choice-brand steaks, roasts and ground beef products were pulled from store shelves because of possible contamination with E. coli.

While the vast majority of people fully recover from a bout of E. coli within a week to 10 days, some people will spend the rest of their lives dealing with the after-effects of the illness.

What is E. coli and where does it come from?

E. coli, short for Escherichia coli, is a type of bacteria commonly found in the intestines of animals and humans. There are hundreds of strains of the bacterium, but E. coli O157:H7 has been identified as the most dangerous to people, producing a powerful toxin that can cause severe illness.

It was first recognized in the United States in 1982, when an outbreak of severe, bloody diarrhea was traced to contaminated hamburgers, leading to the illness to be dubbed as "hamburger disease."

E. coli O157:H7 can contaminate ground beef during the butchering process. If it is present in the intestines of the slaughtered animal, it can get into the meat as it is ground into hamburger.

How does E. coli spread?

While E. coli is most often found in meat, it is not limited to it. The bacteria are also found in unpasteurized milk and apple cider, ham, turkey, chicken, roast beef, sandwich meats, raw vegetables, cheese and contaminated water.

In September 2006, an E. coli outbreak that killed three people and made more than 200 ill was traced to spinach grown in California. Bean and alfalfa sprouts have also been recalled due to E. coli contamination.

Fruits and vegetables that grow close to the ground are susceptible to E. coli contamination if, for example, improperly composted cattle manure is used as a fertilizer.

E. coli, salmonella and cryptosporidium can also be found in unpasteurized juice. Children, the elderly and people with compromised immune systems are encouraged to drink pasteurized juice or boil unpasteurized juice before consuming it.

Once someone has eaten contaminated food, the infection can be passed person-to-person, by hand-to-mouth contact. The bacteria are most often spread from person to person.

What are the symptoms of E. coli O157:H7?

Symptoms — characterized by severe abdominal cramping — can appear within hours, but could also take up to 10 days to show up. Some people may also be afflicted with bloody diarrhea or non-bloody diarrhea. Frequently, no fever is present.

Some people may show no symptoms at all, but can still carry the bacteria and pass it on to people who will become sick.

What are the health effects of E. coli O157:H7?

People who suffer severe E. coli O157:H7 poisoning face a 30 per cent higher risk of high blood pressure or kidney damage, according to a Canadian study released in October 2008. The seven-year study, which included 2,800 citizens of Walkerton, noted medication has stemmed further kidney damage and long-term complications in children. Researchers also found that 88 per cent of participants rated their health at the end of the study as good to excellent. How is it treated?

In most cases, symptoms clear up on their own within five to 10 days. The use of antibiotics is not recommended.

But in a small number of cases, E. coli contamination can lead to a condition called hemolytic uremic syndrome (HUS). This is a life-threatening condition that is treated in hospital intensive care units. It kills three to five per cent of people who come down with it. Some people who recover still have to contend with lifelong complications that can include blindness, paralysis and kidney failure.

How does E. coli get in the water?

According to the U.S. Environmental Protection Agency, the presence of E. coli in water is a strong indication of recent sewage or animal waste contamination.

E. coli comes from human and animal wastes. During precipitation, E. coli may be washed into creeks, rivers, streams, lakes, or groundwater. When these are used as sources of drinking water — and the water is not treated or inadequately treated — E. coli may end up in drinking water.

What precautions can I take to minimize my risk?

Proper food handling techniques can go a long way towards preventing exposure to E. coli. All ground meats should be cooked thoroughly so the centre is no longer pink. Other steps you can take include:

• Refrigerate or freeze meat as soon as possible after buying it and then thaw frozen meat in the refrigerator, not on the counter.
• Place cooked meat on clean plates. Don't re-use dishes that have been in contact with raw meat.
• Use a digital food thermometer when cooking ground beef, which should be cooked to an internal temperature of at least 71 C (160 F).
• Serve cooked meat immediately or keep it hot (60 C or 140 F).
• Clean and sanitize countertops and utensils after contact with raw meat.
• Don't store raw and cooked food together.
• If you marinate meat, don't use the liquid as a dip or to pour over cooked meat.
• Drink only pasteurized milk or cider.
• Drink water from a supply known to be safe. If you have a private water supply (well) it should be tested several times a year.

Since most cases of E. coli contamination are passed from person to person, good personal hygiene is critical to protecting yourself:

• Wash your hands thoroughly and frequently.
• Don't handle food if you are suffering from diarrhea.
• Wash raw fruits and vegetables thoroughly before cooking or cutting them.
• Sanitize food preparation surfaces and utensils.
• Anyone known to be infected with E. coli, should not share dishes, cutlery or glasses with anyone else. Their towels, face cloths and bedding should be washed separately in hot water and bleach.