If you travelled through Toronto's Pearson International Airport last month you may have been exposed to the measles.
Public health authorities are concerned in light of a confirmed case of the illness.
Anyone who was in Terminal 1 on Sunday Jan. 27 from 4 p.m. to 8 p.m. may be at risk.
Those who were near people arriving aboard a Lufthansa flight LH-470 from Frankfurt at 4:08 p.m. may also have been exposed.
Also at risk are those connected to Air Canada flight AC-834 or LH-6829 that left Toronto at 6 p.m. for Montreal.
Authorities urge anyone who may have been exposed to check immunization records and watch for measles symptoms.