Officials say a person contagious with measles inadvertently exposed other people to the illness between 8:30 p.m. on May 22 and 7:30 a.m. on May 23.
Anyone born in or after 1970 who has not received two doses of the measles vaccine, and was at the emergency department during that period is being warned to watch for measles symptoms.
Symptoms usually develop one to three weeks after exposure and may include cough, runny nose, fever, white spots in the mouth and red watery eyes that are sensitive to light.
Anyone who develops symptoms of measles should seek medical attention.
Peel public health is working with the hospital to contact people who were in emergency during that period.
Most people who get measles are sick for about 10 days, and recover completely without treatment.
However, measles can be more severe in infants and adults than in children, and result in pneumonia or encephalitis.