Ontario’s summer health risks explained

A man kayaks in Lake Ontario on a warm sunny day in Toronto on Friday, August 19, 2016. THE CANADIAN PRESS/Nathan Denette

As Ontario enters peak summer season, infectious disease experts are warning that some viruses — from respiratory bugs to mosquito-borne illnesses — thrive in the heat.

While many people may consider winter to be the season when they are most at risk of catching a virus and becoming ill, experts who spoke to CTV News Toronto said that there are many infections that actually peak in the summer.

Dr. David Fisman, a professor of epidemiology at the University of Toronto’s Dalla Lana School of Public Health, said that respiratory infections like rhinovirus, enterovirus and para-influenza are all more common in the summer months and are capable of causing mild illness.

He also said that other more serious health threats, like measles, can “come back” when people make a decision to skip vaccination.

Ontario reported fewer new measles cases this past week; 33 compared to 96 the week before, officials say the outbreak now stands at 2,212 cases since October.

“If we were having this conversation 100 years ago, we would be talking in a time when in Toronto, summertime was the high mortality season, and most of that mortality was from infectious diseases,” Fisman said, emphasizing that deaths have gone down thanks to smart public health measures.

Does climate change play a role?

Mosquitoes and ticks are another concern as climate change expands their reach across Canada.

Dr. Isaac Bogoch, an infectious diseases specialist at Toronto General Hospital, said Lyme disease is already well established in many parts of Canada.

“We do see a fair bit of Lyme in Canada. In 2024 there were roughly 5,000 cases that were identified, but that’s likely a gross underestimate. We’re probably not detecting the majority of cases. So people should be aware that Lyme is out there,” he said. “It’s very easy to prevent Lyme infections… people can just be mindful where ticks live, typically in tall grass or in forested areas, you can wear insect repellent to reduce the risk of a tick bite, and it’s certainly helpful to do tick checks.”

To check if ticks or reported Lyme disease has been reported in your area, the Public Health Agency of Canada has created a free online portal where residents can enter their postal code or the first three letters of their city.

Travelling this summer?

Bogoch also warned that international travellers should prepare for serious infections not found in Canada.

“Mosquito avoidance is key with insect repellent,” he said. “For infections like malaria, many people should be taking tablets to prevent this infection. It can be very severe and even deadly, and it’s important that people are aware that malaria is out there.”

“It’s a really good idea to get professional travel advice,” he added. “There might be infections in other parts of the world that we just don’t see in Canada, and these are largely preventable.”

Fisman echoed those warnings, noting that hotter climates accelerate mosquito activity.

“Mosquitoes bite more when it’s hotter, their biting rate increases and when you have diseases like dengue or yellow fever or chikungunya… that all gets speeded up and outbreaks can happen,” he said.

Hotter weather ‘can increase’ foodborne illnesses

While some people associate summer illness with food poisoning or stomach bugs, Fisman said the risks are “climate sensitive.”

“There are foodborne illnesses like Campylobacter, Shigella and salmonella… and those are climate sensitive,” he said. “Hotter weather can increase the risk of foodborne illness, obviously increase the risk of food spoilage and so forth.”

He also noted the growing concern around antibiotic resistance, pointing to new Canadian research: “My colleague Dr. Derek MacFadden, at University of Ottawa, has actually shown that hotter weather is associated with a surge in antibiotic resistance, which people may not realize.”

How to stay healthy?

Experts say basic hygiene still matters, especially when it comes to respiratory viruses.

“The advice in terms of respiratory infections is the advice for COVID,” said Fisman. “If you’re in a closed, crowded space, you can open a window. You can do activities outdoors if you’re concerned. You (should not) show up and infect other people if you’re sick.”

Bogoch agreed the risks are manageable — if people stay informed.

“Enteroviruses are more common in the summer months, and to no one’s surprise, we’re starting to see a rise in reported enterovirus cases in Canada,” he said.

But he wants to reassure people there is no reason to sound the alarm.

“The key message is, have a wonderful summer. Enjoy. We’re going to be okay.”