The Elgin St. Thomas Public Health Unit says those who attended the "Acquire the Fire" event in Toronto may have been exposed to the measles.
One of the people who was at the event at the Queensway Cathedral in Toronto on February 6 and 7 has been diagnosed with the measles.
The measles is an air-borne illness and can quickly spread to unvaccinated people.
Anyone who was at the event is asked to check their immunization records to ensure they are protected.
If you have not been immunized, you should stay at home for 21 days since attending the event. It is advised that you avoid contact with babies, pregnant woman and those with weak immunize systems.
Symptoms of the measles include rash and fever, cough, runny nose and inflamed eyes.
Anyone who attended the event and develops these symptoms is asked to contact their doctor or alert all health care providers before visiting any walk-in clinics or hospitals.
There are now over a dozen confirmed cases of the measles in Ontario.
An outbreak of over 100 cases has been linked to California's Disney Land.