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As almost Canadians have heard by now, the country is knee-deep in a flu outbreak. Just about everyone knows someone who’s been felled by illness.

But as the virus circulates, so too does the confusion about what is really flu, and what is what we all like to call "stomach flu."

In fact, there’s really no such thing as "stomach flu." Flu, or influenza, is an illness that affects the respiratory system, not the stomach. Infectious diseases expert Dr. Neil Rau says it just so happens that while we’re grappling with a flu outbreak, the country also has an outbreak of gastrointestinal infections called norovirus.

"There’s huge confusion between 'stomach flu’ – which is Norwalk virus or norovirus – and the flu," Rau told CTV’s Canada AM Thursday.

"Norwalk does not actually cause fever. That’s a big distinction. ‘Stomach flu’ is lots of diarrhea, vomiting, belly cramps, but no cough, no sore throat or runny nose."

Once known as "winter vomiting disease," noroviruses tend to spread in the winter, bringing on illness suddenly, and then leaving just as quickly. Rau says the typical norovirus illness lasts one or two days.

"It can even be as short as 12 hours. If your illness lasts more than three days, you do not have Norwalk," he says.

Influenza, on the other hand, typically leaves patients sick in bed for a week with fever, cough and chills. Nausea and vomiting are rare in adults, but the fever of flu can sometimes bring those symptoms on in children.

As for the current flu outbreak, Rau believes the worst of it is finally starting to come to an end.

"We’re probably at the height of it in most of Canada and past the height in many places," he said. "It doesn’t mean there’s no virus circulating; we’re just seeing fewer and fewer cases."

Rau points out that flu is not the only respiratory bug that circulates in the winter, so not everyone who comes down with flu symptoms actually has influenza.

"For every person who has a flu-like illness, only one of three at the height of flu season actually has the flu," he says.

With all kinds of other influenza-like illnesses out there, it might explain why some people seem to "get" the flu from the flu shot. In fact, in many of these cases, the vaccine may have protected them from influenza; they just had the bad luck to pick up something else altogether.

Of course, the flu vaccine, like every vaccine, doesn’t offer 100 per cent protection to 100 per cent of the population. A recent report from doctors running Canada’s flu surveillance network found that Canadians who got a flu shot this year cut their risk of being hospitalized with the flu by about half.

While that might not sound great, for flu -- a virus with many strains that have the notorious ability to change themselves as they travel -- that’s pretty good.

"It’s not a perfect vaccine but it does offer fairly good protection. And it’s actually a very safe vaccine," says Rau.

"Especially if you have the risk factors for a bad outcome from the flu, you definitely should get it. Even if you don’t have risk factors, if it does cut your chances of getting the illness, so that’s still worth something," he adds.

Rau says this year has been a good example of why it’s better to get the vaccine before the first wave of illness hits and makes headlines. But he also thinks there’s still benefit to getting the shot this late in the season.

"I think if people have risk factors for complications, they should absolutely still get the vaccine even if we’re past the peak, because there’s still some virus out there."