Toronto Public Health has recorded four cases of measles in two children and two adults within the past week.
And a department official admits there are likely more cases in the city, because none of the infected people have recently travelled outside the country.
The measles virus does not regularly circulate in Canada.
Cases are typically only reported when an unvaccinated person gets infected abroad and brings measles back to Canada, or when an infected person travels here and spreads the virus.
Sometimes those imported cases don't lead to local spread. But in other cases, they can trigger large outbreaks, such as last year's epidemic in British Columbia in which more than 400 people became infected.
Dr. Lisa Berger says Toronto Public Health is investigating the four cases to try to determine how the infected people contracted the virus.
Measles is best known for triggering a widespread red rash. But the virus can make people who contract it — especially young children — very sick.
In the United States, about 28 per cent of the young children who contracted measles between 2001 and 2013 ended up in hospital. Complications can include pneumonia, permanent brain damage and deafness.
Measles can also be fatal. While most survive, it's estimated that between one and three children out of every 1,000 who are infected will die.
Berger says people born after 1970 who haven't had two doses of measles vaccine should get vaccinated.
Measles was widespread in Canada before the vaccine was introduced in 1970. People born before that date are believed to be immune because they would have been infected previously.
Berger says none of the four people who have been diagnosed in the past week had the requisite two doses of measles vaccine.
