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: