You can't see it, smell it or taste it. But food or water tainted with certain strains of E. coli 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.

The latest E. coli outbreak in Canada led to a recall of over 1,500 product lines that were packaged at XL Foods in Brooks, Alta. The recall began Sept. 16, twelve days after two positive tests.

A common illness

It was the well-known Canadian version, E. coli O157:H7, which contaminated the water system in Walkerton and apparently the XL Foods plant.

While the Walkerton outbreak was unusual, health authorities in Canada normally deal with a few thousand cases of E. coli illness a year. In the U.S., it's estimated that about 73,000 people a year are sickened by the bacteria and 61 die.

In 2006, tainted spinach from California led to the deaths of five Americans and sickened over 200, including a Canadian woman.

E. coli O157:H7 can contaminate ground beef during the butchering process. (iStock)

In another 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. Undercooked ground beef is one of the most common sources.

In 2012, the outbreaks included Fredericton, N.B. in July and Miramichi, N.B. in April.

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?

Escherichia coli, its full name, 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 dangerous to people, producing a powerful toxin that can cause severe illness.

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

E. coli O157:H7, E. coli O104:H4 and other deadly strains belong to a family of bacteria that's evolved since the 1960s, when scientists believe E. coli and another bacteria, shigella, met and swapped genes. This created a form of E. coli that secretes the dangerous shiga toxin.

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.

Bean and alfalfa sprouts have also been recalled because of 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 from one person to another person by hand-to-mouth contact. The bacteria are most often spread 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 be afflicted with bloody diarrhea or non-bloody diarrhea. seizures or strokes may occur. 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?

An E. coli O157:H7 bacterium under a microscope. (iStock)

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.

In severe cases, people may die.

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 because the bacteria creates a toxin in its cell and if you kill the cell with antibiotics the toxin gets released into the bloodstream.

In a small number of cases, E. coli contamination can lead to hemolytic uremic syndrome (HUS), 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 happened in the deadliest E. coli outbreak, in Germany in 2011?

In summer 2011, a different and mysterious strain of E. coli led to the deaths of 51 people in Germany and one in Sweden and one in the U.S.. It sickened about 4,000 people, most of them in Germany.

Czech scientists in Brno test vegetables for E. coli bacteria following a deadly outbreak in Germany that killed 53 people and sickened about 4,000 in 2011. The source of the bacteria was eventually identified as fenugreek seeds. (Reuters)

This deadlier strain of E. coli, O104:H4, has been particularly virulent in that it has also led to hundreds of cases of HUS.

The source of the E. coli at first eluded health officials, who advised consumers not to eat cucumbers, tomatoes, lettuce or vegetable sprouts – all of which were investigated as possible carriers of the bacteria.

But after more than a month of searching, the problem was traced to sprouts from an organic farm in the northern German village of Bienenbuettel. The farm grows a wide variety of sprouts, including alfalfa, onion and radish, and officials weren't immediately sure which kind caused the outbreak. Sprouts are often eaten raw in salads and sandwiches.

Then in July, the European Food Safety Authority said one lot of contaminated fenugreek seeds from Egypt was probably the source. Fenugreek leaves are commonly used as an herb and in curry. The seeds are often sold dried, and if they are contaminated with E. coli, the bacteria can survive for years.

E. coli O104 is a shiga toxin-producing E. coli, or STEC, that is especially vicious because it doesn't take many bacteria to cause infection.

The bug gets into the stomach and then attaches to the intestinal wall and secretes a toxin that destroys red blood cells and shuts down the kidneys.

Once securely inside the gut of one person, the bacteria can then start spreading person to person through the fecal-oral route. That happens when traces of feces on the hands get passed on, which is why hand-washing is so important.

The E. coli outbreak in Europe has mostly affected healthy adults, not children or the elderly.

It seems a higher rate of people progress to HUS from the initial condition of bloody diarrhea, said Brett Finlay, a professor at the University of British Columbia who studies pathogenic strains of E. coli.

How can exposure to E. coli be prevented?

Proper food handling techniques can go a long way toward preventing exposure to E. coli. All ground meats should be cooked thoroughly. You should also:

• 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.
• Not store raw and cooked food together.
• When marinating meat, not 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 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 have 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. Their towels, face cloths and bedding should be washed separately in hot water and bleach.

(CBC)