Once the care home has gone 48 hours without a new case, the outbreak can be declared over, he said.

Stanwick has said the strain of norovirus that began sweeping through the home on July 11 was likely “the garden variety,” and not a new, nastier strain that was detected in Australia and the U.S. last year.

But he added it’s concerning that so many people became ill so quickly — 40 became ill within 24 hours of the first case. The quick spread also suggests the virus came from a common source, perhaps from even one sick staff member or visitor, or perhaps from contaminated food.

Infectious diseases expert Dr. Neil Rau says while the virus may not have been a new strain, it might still have been somewhat altered.

“To me, it raises the possibility that this could be a slight change in the strain of norovirus from what we had seen during the past season,” he told CTV’s Canada AM Thursday. “But we need more testing.”

Rau says noroviruses are not usually deadly, typically bringing on vomiting, cramps and diarrhea over the course of a few days. But he says, like any virus, noroviruses are constantly mutating so they can infect new hosts.

The viruses tend to spread in confined places, such as cruise ships and nursing homes, although outbreaks in the summertime are rare, he said.

Most people are able to recover from the stomach bug after a few days, but for infants and elderly people – especially those with chronic illnesses – the virus can take a serious toll. Still, Rau says it’s unlikely the virus killed the nine residents, though it may have contributed to their deaths.

“Nine deaths among 150 would be exceptional,” Rau said. “The death rate is usually more like 1 in 10,000, to 1 in 20,000.”

Both Rau and Stanwick says it’s likely that the residents who died underlying conditions and the severe dehydration caused by the norovirus infection contributed to their deaths, but was not the main factor in their deaths.

Stanwick said the health authority will now try to determine what sparked the outbreak and whether anyone who may have had the virus was handling the facility’s food.

Rau notes that noroviruses are notoriously contagious because they are so hardy. An infected person can shed billion of virus particles and typical disinfectants –even those routinely used in hospitals – are often not enough to kill them on surfaces. And once a surface is contaminated, many people can become infected.

“You don’t need a lot of the virus to get sick; 10 virus particles are enough to make you become ill,” Rau said. “You can be infected by picking up just a few of those virus particles and putting them in your mouth.”

“The other way you can get it is through food. You might have a food handler in the kitchen who is a little bit ill, contaminated the food of many residents and infects many people at once.”

Rau says it’s important to use proper infection control measures such as wearing gloves and washing hands once an outbreak is discovered. But he says often by then, the outbreak is already close to hitting its height.

“I think that this one is already under control,” he said. “They’re still going to have new cases, but the number of new cases per day is dropping and that’s what you want to see.”

With reports from The Canadian Press