Health officials in Manitoba say they might never solve the mystery of how a two-year-old boy became infected with E. coli bacteria.

"I think it's possible that we will never discover the source of the E. coli 0157 that produced this case," Dr. Tim Hilderman, acting director of communicable disease control, said Wednesday.

They have ruled out food, water and even the daycare centre of Brooks Oliver, an Arborg-area toddler who died in late June.

His sister Brianna also became infected, as did an adult from their area. Those two survived.

"It was very hard to see a boy always playing and into something with his sister just lying still and being in the fight of his life," Lynette Oliver, the children's mother, told CTV Winnipeg.

She said the tragedy began on Friday, June 17.

Brooks had a fever, and Lynette thought he might be teething.

"I didn't think too much of it until the bloody diaper came off, and that's when we went right away to Arborg and then off to Winnipeg," she said.

"And then Wednesday morning, that's when my daughter started with the bloody diarrhea," she said. "What was running through my mind was, 'Oh my goodness, I'm going to have two in ICU, two on dialysis'."

Lynette said the family was around Brooks when he died. His is the first E. coli death in Manitoba in more than five years.

The strain of E. coli that caused the illness is the same type that hit Walkerton, Ont. in 2000.

Seven people died and more than 2,000 were sickened in that case, which led to a public inquiry and criminal convictions.

The Walkerton tragedy happened after the town's water supply became contaminated with E. coli.

Contaminated food, especially ground beef, is another way people can become infected. Fruits and vegetables contaminated with manure can also be dangerous.

Infected people can pass the bacterium to others. Some infected people might not even show symptoms. Experts say random cases where only a few people are infected are the toughest mysteries to solve.

Most infected people recover within five to 10 days, but a small percentage die. In the wake of Walkerton, studies found some previously healthy people experiencing kidney and high blood pressure problems four years after becoming infected.

With a report from CTV Winnipeg's Rachel Lagace