Health authorities were warning all fall that this could be a bad flu season, because of the type of virus that is most commonly infecting people and the fact that this year's flu vaccine doesn't offer optimum protection against it.
Other viruses are circulating too, adding to the general confusion about what influenza actually is and how much of the illness being seen at the moment is due to flu viruses.
We asked some experts to explain what they are seeing this flu season:
Question: Is this year worse than other recent years?
This seems to be shaping up to be a pretty active influenza season.
It started early, with a lot of virus around in December.
Flu seasons that start to pick up before Christmas can take off, because the multi-generational mixing at family gatherings is a perfect way for influenza to transmit from toddlers and tykes to grandmas and grandpas — and mums and dads as well.
But when flu peaks over the holidays, a few factors can combine to make it seem like there is a lot of it around.
Family doctors may work reduced hours, so people who do seek medical care may turn to hospital emergency departments. Or people who get sick when they are away from home visiting family may have to go to an emergency room because they don't have a local doctor.
Hospitals operate with reduced staffing levels over the holidays — doctors and nurses want time off too — so it doesn't take much to stress the system, suggests infectious diseases expert Dr. Allison McGeer.
McGeer, who heads infection control at Toronto's Mount Sinai Hospital, says so far this year influenza rates seem to be on a par with last year's, though the season peaked a little later in 2013-14.
The numbers are worse than two years ago, but better than three seasons ago in terms of hospital admissions for influenza thus far, she says.
"So yeah, it's a busy year."
Question: Is there something different about this flu season? Isn't there a virus circulating that isn't in the flu vaccine?
Almost all the influenza illness in Canada to date this year has been caused by a family of influenza A viruses called H3N2. When those viruses dominate, doctors expect a severe flu season. For reasons that aren't fully understood, H3N2 viruses are really hard on the elderly.
The flu shot has an H3N2 component in it; it does every year. But when experts met last February to select the strains for this year's flu shot, a particular type of H3N2 was dominant. Since then, new varieties have emerged and they are different enough from the vaccine virus that it is assumed the flu shot will only offer modest protection against them.
Even in the best years, flu vaccine doesn't offer complete protection. Some people who get a shot will still get sick. In a year like this one, when there is a mismatch between the circulating viruses and the vaccine virus, you would expect to see more infections in vaccinated people.
But that's not the biggest driver of flu infections this year. After all, most people in Canada don't get a flu shot. Most years fewer than one in three people are vaccinated against flu.
For the unvaccinated, the changes in the H3N2 viruses mean they too are more likely to get sick this year. The antibodies their immune systems would have generated after previous bouts of influenza won't protect as well against the new viruses.
In fact, the unvaccinated people actually make up the bulk of what influenza expert Dr. Danuta Skowronski describes as "a tidal wave" of flu activity the medical system is seeing this year.
Question: So a vaccine mismatch doesn't mean these viruses are more dangerous?
In a word, no.
H3N2 viruses are especially hard on a vulnerable portion of the population, older adults. And the ones circulating now are different enough that more people may be at risk of getting infected.
But the viruses aren't inherently more dangerous.
Question: How do I know if I have the flu?
Most people who contract influenza don't get sick enough to need medical care. And most of those who do see a doctor are not actually tested — the diagnosis is made based on symptoms.
It can be easy to mistake an infection caused by one of the multitude of other cold-causing respiratory viruses for influenza.
But there are some classic symptoms that can help you figure out if what you have is influenza, says Skowronski, who is with the British Columbia Centre for Disease Control.
For starters, flu comes on abruptly.
"You go from feeling well to feeling like death warmed over within a few hours," Skowronski explains.
Most people who have influenza will have a fever, which is part of the immune system's response to infection. Senior citizens may not, because their immune systems are not as effective as they were when they were younger.
Muscle aches and lethargy are also classic features of influenza.
"With other things like rhinovirus, you kind of feel under the weather. You've got a runny nose or a sore throat or congestion. But you can push on with your day," Skowronski says.
"With influenza you feel completely depleted. You feel really like the energy has been sucked out of you. And you need to take to bed."
Question: If I don't have flu, what do I have?
Lots of viruses cause what medicine refers to as ILI — influenza-like illnesses. Rhinoviruses, which are the most common cause of colds. Parainfluenza viruses. Adenoviruses. Some coronaviruses. And respiratory syncytial virus, also known as RSV.
Winter is also peak season for these bugs. And right now, there is a lot of rhinovirus and RSV activity, says Dr. Doug Sider, medical director for communicable disease prevention and control at Public Health Ontario.
Sider says this co-circulation of viruses can make things confusing for the public.
"If you're under four (years of age) and you've got ILI, you've got probably a one-in-four, one-in-three chance that it's influenza versus one of the other respiratory viruses," he says.
"So I think this is a real challenging season. Because as you can imagine, there's all sorts of mixed conclusions from this."
Question: If I have the flu, what should I do?
You probably won't have much choice. The only place you'll want to be is in bed.
Skowronski prescribes rest and rehydration, to replace the fluids you lose through sweating out the fever. Most people will come through a bout of flu without any problems and without the need for medical help, she says.
"It's miserable illness. It's nasty.... But they're going to recover fully without any intervention. They should stay home so they're not clogging the medical care system for those who actually truly do need it — which are the high risks (groups)."
Give yourself time to recover, she says. And taking yourself out of circulation contributes to limiting the spread of influenza viruses.
People with flu are most infectious when they have the most symptoms. If you are sneezing and coughing at work or on public transit at that point, the people sharing those spaces with you won't appreciate the fact you are trying to soldier through your illness.
The period of contagiousness lasts for about five days after the peak of symptoms, she says.
"That's why we say: 'Fever and cough, take the week off.'"
Question: When should I seek care? How do I know if my child or my elderly parent's infection requires a doctor's care?
Flu can be life threatening, with the very young and the elderly more at risk of severe illness than young adults.
Anyone who has a high fever that won't abate or is having difficulty breathing should get medical care.
"Those are hallmark signs you need help. Never should you ignore difficulty breathing," Skowronski says.
With young children, she says parents should trust their gut instincts.
"If the child has high fever, doesn't seem to be turning the corner, isn't eating or feeding, then they may want to get that child seen as well. Because the very young are more susceptible to more severe outcomes."
With the elderly, a severe flu infection may initially be harder to spot, because fever may not be present. Look for someone who is having trouble bouncing back, who isn't eating or drinking fluids much.
"It may be a slow case of 'the dwindles' you might call it," Skowronski suggests.Suggest a correction