As flu season fades, spring and summer viruses emerge. What are they?

Click to play video: 'Health Matters: Norovirus spreading in Ontario, Alberta'
Health Matters: Norovirus spreading in Ontario, Alberta
WATCH: Norovirus spreading in Ontario, Alberta – Apr 30, 2024

As Canadians bid farewell to the aches and chills of the season of respiratory viruses, like influenza, a new question looms: what other microbial menaces will appear as days grow warmer?

As the transition from winter to spring unfolds and people begin to gather outdoors, it’s crucial to remain vigilant against a range of viruses that thrive in the warmer months, experts warn. While some infectious agents, like the common cold and flu, may recede as the days get warmer, others persist year-round or even intensify as temperatures rise.

In reflection of the recent flu season, Dr. Colin Furness, an infection control epidemiologist and assistant professor in the faculty of information at the University of Toronto, notes it was quite “moderate.”

“It peaked in the middle of the winter and influenza A is almost gone and influenza B peaked recently, which is a little late, but on the way down, too,” Furness told Global News.

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At the end of April, he said, influenza can be quite substantial, but this particular year it’s not.

Click to play video: 'Health Matters: Researchers create virus detection network'
Health Matters: Researchers create virus detection network

National flu activity has steadily decreased over the past few weeks, according to the latest data from the Public Health Agency of Canada’s (PHAC) Flu Watch. Influenza A continues to decrease and is lower than influenza B, which is also decreasing, PHAC said.

As flu season fades, spring and summer viruses emerge. What are they? - image
Public Health Agency of Canada (PHAC)

And as influenza season comes to an end, here is what Canadians can anticipate the emergence of other viruses and bacteria in the warmer months.

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Enteroviruses are a diverse group of viruses comprising over 100 types and tend to be more common during the summer months. They can manifest in various illnesses, ranging from mild cold-like symptoms to more severe respiratory and neurological conditions, according to the BC Centre for Disease Control.

The health authority said most people who get infected by enteroviruses will not get sick at all. But infants, children and teenagers are more likely to get infections partly because they have not been previously exposed to as many kinds of enteroviruses as adults and have not developed immunity to these viruses.

“Enteroviruses pop up as soon as we get into the summer,” said Dr. Gerald Evans, an infectious disease specialist at Queen’s University in Kingston, Ont. “These viruses are growing in water and as people go swimming in lakes and rivers and streams…. this is where enteroviruses hang out.”

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Because of this, waterborne transmission is a significant route by which enteroviruses can spread, especially during the summer months when people are more likely to engage in water-related activities.

A classic example of enteroviruses is polio, Evans said.

Click to play video: 'Viral infections still circulating'
Viral infections still circulating

“Back in the day before there was a polio vaccine, parents and families used to fear in the summer because we knew that (was) when polio would happen,” he said. “And of course, nowadays we have a really great vaccine against polio, so nobody should ever get polio anymore, particularly here in Canada.”

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During the summer months, an enterovirus that tends to emerge is hand, foot and mouth disease, to which children are more susceptible due to their reduced immunity.

“This is generally not a super serious disease, but it’s certainly painful,” Evans said. “You develop blisters on your hands and the soles of your feet and in your mouth. And sometimes those can become secondarily infected with bacteria, which can cause hospitalization.”

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And then there is the dreaded common cold that some people get in the summer months.

Although less common, the rhinovirus — commonly referred to as the common cold — is classified as an enterovirus and can still occur during the warmer months.

“The common cold is really common in the spring,” Furness said. “And to me, that is a bit of a head-scratcher, but there’s probably more of it around in the spring, we don’t completely know. Summer colds are a little more unusual, but it can happen.”


Although cases of COVID-19 spiked alongside other respiratory illnesses such as influenza this winter, a discernible seasonal pattern has yet to be established.

“COVID is not seasonal yet and may never be,” Furness said.

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Unlike influenza or rhinovirus, which thrive in specific conditions such as clustering indoors and dry air, he said, COVID-19 doesn’t rely on such ideal environments to spread. Its high contagion rate is more akin to measles than the flu, making it a vastly different challenge.

Click to play video: 'Health Matters: Canada’s public health agency studying COVID-19 seasonality'
Health Matters: Canada’s public health agency studying COVID-19 seasonality

“COVID does not require ideal conditions, which is a huge determinant of seasonality,” he said. “COVID is out there right now.”

Current COVID-19 numbers remain low across Canada, according to nation wastewater surveillance.

Furness said that because of its highly contagious nature, the virus doesn’t adhere to seasonal patterns. As spring and summer roll in, the risk of contracting it remains very real. And because we aren’t tracking the virus either these days, you may pick up a strain that is “a bit more infectious,” he said.

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“You should be prepared. If you’re going to be in a really crowded place and you don’t have really good knowledge about exactly what the air quality is like, you should be prepared to get COVID,” Furness warned.


Norovirus is a highly-contagious virus that causes acute gastroenteritis, commonly known as the stomach flu or winter vomiting bug.

Norovirus spreads primarily through the fecal-oral route, meaning it is transmitted through contaminated food, water, surfaces, or direct contact with an infected person. Once ingested, the virus targets the lining of the stomach and intestines, leading to inflammation and the characteristic symptoms of norovirus infection, including nausea, vomiting, diarrhea and stomach cramps, according to PHAC.

“Norovirus is quite hearty and hugely contagious and it does like warmer weather,” Furness said. “It tends to be warmer weather because the environment is milder, and it can grow and proliferate.”

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Click to play video: 'Cruise ship with suspected Norovirus outbreak headed for Halifax'
Cruise ship with suspected Norovirus outbreak headed for Halifax

Norovirus can pose a particular risk for travellers, especially those vacationing in areas where hygiene standards may differ or where large groups of people gather, such as cruise ships, Evans said. Cruise ships, in particular, have been associated with norovirus outbreaks due to the close living quarters and communal dining areas onboard.

“We all hear about those all the time, that norovirus sweeps through some luxury cruise line,” he said.


Arboviruses, also known as arthropod-borne viruses, refer to a diverse group of viruses that are transmitted via mosquitoes, ticks or sandflies.

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“One of the viruses that we see reappearing, typically in the mid-to-late summer period is West Nile virus, which is carried by mosquitoes,” Evans said.

While many people infected with the virus may not develop symptoms, others may experience flu-like symptoms such as fever, headache, body aches and fatigue.

“In Canada, it’s more common to see rises in cases in Western parts of Canada, but we still see it here in Ontario,” he said.

West Nile virus is not native to North America, but it arrived in the late 1990s. Since then, it has become established in various regions across Canada. Periodic outbreaks of West Nile virus occur during the warmer months when mosquito activity is at its peak, Evans explained.

Click to play video: 'A new drought-resistant mosquito carries West Nile virus into Alberta'
A new drought-resistant mosquito carries West Nile virus into Alberta

“West Nile just produces febrile illnesses and people for the most part wouldn’t even know they got it, they would get a fever in the middle of summer and go, ‘Oh, well, I guess I got something,’ and then it goes away,” he said.

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Another arbovirus that circulates during the summer months is transmitted by ticks and is known as Powassan virus.

Powassan virus is named after the town of Powassan, Ont., where it was first identified in 1958.

It is primarily transmitted to humans through the bite of infected ticks, particularly the Ixodes species, including the black-legged tick (also known as the deer tick).

“It’s carried by groundhogs, so it is really rare,” Evans said. “We would probably see one maybe two a year, in the province of Ontario, and some years no cases at all.”

How to stay safe

While the summer creates opportunities for other viruses to emerge, Evans and Furness emphasize that the best defence against viruses that surface during the warmer months is simple: basic handwashing skills.

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“Hand hygiene is so incredibly important. One of the many things that we always encourage people to do, because it’ll help to prevent the transmission of a lot of different things,” Evans said.

Furness suggested carrying hand sanitizer when you’re out and about, whether it’s at the beach, park, or carnival. This simple practice can effectively prevent viruses from entering your body.

“If you want to keep things like this at bay, especially if you have young kids, you can get in the habit of sanitizing their hands when outside,” he said.

“It’s affordable and it’s incredibly effective.”

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