Crews were called to a townhouse complex in the area of Downey Road and Teal Drive at around 12:30 p.m. on Wednesday.
Officials said the lit candle ignited nearby combustibles.
Smoke and flame damage could be seen around the top-floor window, but Osborne said the fire did not spread to the attic and roof.
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Everyone inside was able to get out on their own. Two people were treated for smoke inhalation and sent to hospital as a precaution.
A cat was also treated for smoke inhalation and taken to the University of Guelph Vet Centre for treatment.
One nearby resident said university students may have been renting the unit.
A damage estimate was not provided.