TORONTO - After a couple of relatively mild seasons, this year the flu is really making headlines.

But there are a lot of viruses making the rounds this winter. It's easy to mistake one for another — and hard even for doctors to tell some of them apart at times.

This is, after all, the sick season. The cold-and-flu season. A time for the spread of viruses that make folks wheeze and sneeze and feel like crap. It's also a time for viruses that tether you to the bathroom.

Let's take a look at some of the bugs that are making people sick these days.

First off, there's influenza. Known more commonly as the flu, it's a respiratory illness. It infects the lungs, not the stomach. (More on this later.)

Flu's signature is that it comes on fast and hits you hard. It's symptoms are like those of a cold, but generally worse. So congestion, coughing, runny nose — that whole complex of symptoms is something you can expect from the flu.

In addition, influenza sufferers often complain of fever, headache, muscle aches, profound fatigue. This is the one that sends you to bed for several days on end. And it can take several weeks to feel fully recovered. Influenza can also open the door to pneumonia, an infection of the lungs.

This year flu has really been making itself felt in many parts of the country, though in some places it is already past its peak. (Flu seasons typically show a very sharp curve of infections, then taper off.)

Infectious diseases specialist Dr. Michael Gardam says this year's flu activity in Toronto was explosive, surging from normal for the time of year in the week before Christmas to serious levels the following week.

"Normally there's sort of a step-wise increase and this was a very dramatic increase," says Gardam, head of infection control for Toronto's University Health Network.

But that wasn't the case everywhere. Manitoba, for instance, has had a "busy-ish" flu season, according to the province's chief public health officer, Dr. Michael Routledge. But he says another virus caused quite a bit of illness in Manitoba before influenza took hold.

That other virus? RSV.

Respiratory syncytial virus is one of the viruses that falls under the umbrella of "influenza-like illnesses." It's a crowded category, including coronaviruses, rhinoviruses, adenoviruses, para-influenza (which is not influenza) and human metapneumoviruses.

It was a really bad fall for rhinoviruses, which are often called the cause of the common cold. But rhinovirus infections started to decline sharply around mid-October and by mid-December influenza and RSV were taking over.

RSV isn't flu, but like influenza it attacks the lungs and causes the same panoply of symptoms experienced with influenza.

It's a common infection, and according to last week's FluWatch report from the Public Health Agency of Canada, it's on the rise at the moment in Canada. That's a national estimate; the mix of bugs and where they are in their cycles varies widely across the country.

The Public Health Agency doesn't estimate how many hospitalizations or deaths RSV causes it Canada, but notes that in the U.S., about 100,000 people are hospitalized with RSV infections and 4,500 die each year.

The virus tends to hit children hard, but adults too can be infected. And as with many respiratory viruses, infection once does not confer long-term immunity. In other words, you can catch it multiple times.

Here's another thing about RSV: If you have what feels like a bad cold or the flu, there's no way to know what is causing your symptoms unless you get tested. And most people don't see a doctor for these illnesses; they don't need to.

"There really is a lot of overlap between the two of them so there really isn't enough of a difference that you could tell," Gardam says.

Where there is a difference — though it's not one the public always grasped well — is in the case of norovirus infections.

Once known as "winter vomiting disease," noroviruses attack an entirely different system — the gastrointestinal tract.

Infection with these viruses doesn't cause cold-like symptoms or pneumonias. These guys trigger vomiting or diarrhea, or both.

People sometimes use the term "stomach flu" but it is a misnomer. There is no link between influenza and noroviruses.

While small children sometimes vomit when they are infected with influenza, if you are vomiting and/or suffering from diarrhea at this time of year, it's probably a norovirus infection.

Doctors describe norovirus infections as mild, because infections don't last more than a day or two. But that's generally 24 to 48 hours of real discomfort.

"It doesn't seem mild when you're sick," acknowledges Natalie Prystajecky, an environmental microbiologist at the British Columbia Centre for Disease Control who specializes in noroviruses.

While noroviruses come around every winter, some years are worse than others. That's because every two to four years or so a new strain emerges that seems better able to slip past immune systems.

There's a new one making the rounds globally at the moment, the Sydney strain (named thus because it was first spotted in Australia). It has caused huge outbreaks in Britain this winter, and has been responsible for a number of outbreaks in British Columbia too.

Officials in Alberta last week said they are also having an active norovirus year. But in Ontario, the season really hasn't taken off, says Dr. Jonathan Gubbay, a medical microbiologist with Public Health Ontario.

"Right now the season is playing out like a normal one," he says.

SEE: The 10 things you need to know about norovirus:

Loading Slideshow...
  • Where Does It Come From?

    The virus is spread through an infected person's feces or vomit, and often by unwashed hands. "It's not that it's in food, but more often than not, it's in the environment," says Dr. Gerald Evans, a professor of medicine, biomedical and molecular sciences at Queen's University, and the medical director for infection prevention and control at Kingston General Hospital. "You don't want to know how much stuff from people's intestinal tracts is all over the environment."

  • Where Can You Get It?

    The most common places for norovirus to spread are residences where many people are living together — nursing homes, for example, or cruise ships.

  • What Are The Symptoms?

    The clinical syndrome is characterized by nausea, vomiting and diarrhea, though for some, it can also include a fever and abdominal pain. It comes on very suddenly — usually within 10 hours of transmission — and lasts one to three days. After three days, it's no longer contagious.

  • Who's At Risk?

    Anyone can get norovirus, but it can be a particularly bad illness for the very young, and the very old. "Healthy people who get it feel awful, but they recover quickly," says Dr. Evans. "The problem is when it combines with other ailments. For very old people who might have other health problems, it can have serious effects, while very young people can dehydrate much more quickly." Interestingly, the virus particularly likes people with the blood group O, which constitutes about 45 per cent of the population. This is thanks to the receptor the virus attaches itself to. If you have another blood type, you can still get norovirus, but the disease will likely not be as severe.

  • What Can You Do If You Contract Norovirus?

    "Lay low and wait for yourself to get better," advises Dr. Evans. There's no treatment, and while medical professionals advise keeping fluids up, Dr. Evans acknowledges this can be difficult, given the nature of the illness. "We want people to try to hydrate themselves as best they can, but it can hard," he says. "Because it's usually just one day, the situation doesn't get too dire, but every so often, we see perfectly healthy adults coming into the emergency room for intravenous fluids to get hydrated again."

  • What Can People Do To Protect Themselves?

    You'll want to practice good hygiene in order to reduce the possibility that you'll ingest the virus, recommends Dr. Evans, and of course, try to avoid being in a circumstance where you can get the virus. "We really encourage handwashing, but I won't tell you will absolutely not get sick if you wash your hands," he says. "Viruses are tiny little particles, and it doesn't always matter how fastidious you are at cleaning things — they get everywhere."

  • Why Is It Such A Concern?

    The biggest problem, notes Dr. Evans, is that the virus is very transmissible, and can easily pass from person to person. It's also quite the trial on your health. "It's very traumatic," says Dr. Evans. "It's amazing how fast it starts, and it's amazing how bad you feel for at least a day or two. Most people who get it would rather have anything else."

  • What Is The Difference Between Norovirus And The Flu?

    The flu is an entirely different illness than norovirus. As Dr. Evans explains, the flu takes place in the respiratory system, while norovirus is a gastroenterological illness. Besides the lack of cough and cold in norovirus, it also has a much short lifespan: Three days vs. the flu's five to seven-day stint.

  • What Is The Difference Between Norovirus And Norwalk?

    Norwalk is an old term for norovirus, says Dr. Evans. "Viruses are always named geographically, and the first norovirus was discovered in Norwalk, Ohio, so it was given that name. It's been since changed to give the group of viruses the name 'norovirus.'"

  • Are There Any Long-Term Effects?

    "As far as we know, there are no long-term effects — it's a very self-limited illness," says Dr. Evans. "The biggest, scariest thing about noro is that you're never immune to it, because there are a bunch of different strains. Once you get it, you can pretty well guarantee you will get it again."

Also on HuffPost: