Dr. Rita Shahin, one of the city's associate medical officers of health, said Tuesday that officials have become aware of 16 cases in a six-week period. That's well above the three or four cases that officials would usually expect to see.
"We normally begin to see a spike in Legionnaire's disease starting in August and we wanted to let physicians — particularly emergency room doctors — know about this increase and to be aware, so that they test people appropriately that are coming in with pneumonia," Shahin told CBC News in an interview.
The Toronto Public Health website says that individuals become infected by breathing in air that is contaminated with droplets containing a bacteria known as Legionella pneumophila.
People who fall ill from the bacteria will display symptoms similar to pneumonia. Those could including fever, chills, a cough, as well as muscle and headaches.
In the current spike of cases that Toronto officials are observing this summer, there is no common link between them.
Dr. Michael Gardam, a Toronto-based infectious disease specialist, said that there is a general increase in the number of Legionnaires' cases in North America.
But he said there is no consensus at the moment as to why doctors are seeing an ongoing spike.
"There’s a lot of theories going around that perhaps this is related to [the fact] we've had wetter summers, we've had global climate change and that certainly bacteria do take advantage of things like that and we may be seeing changes," Gardam told CBC News on Tuesday.
"It also may just be on the increase because it’s on the increase and we actually don't have a good explanation why."
In 2005, an outbreak of Legionnaires' disease at a Toronto nursing home led to the deaths of more than 20 people.