Up to 170 TTC employees have been forced into self-isolation after a mechanic at one of the transit agency's bus maintenance facilities tested positive for COVID-19.

Toronto Transit Commission spokesperson Stuart Green confirmed on Thursday morning that a TTC mechanic "who does not work on frontline service maintenance" had tested positive for the 2019 novel coronavirus on Wednesday.

The affected employee is said to have worked only one day at Duncan Shop, within the commission's Hillcrest Complex at Bathurst Street and Davenport Road, since returning home from travel on March 11.

A #TTCmechanic who does not work on frontline service maintenance is our first positive case of #COVID19 . Approx 130-170 employees are impacted by a self isolation and monitoring directive. 1/2

— TTCStuart (@TTCStuart) March 19, 2020

Green noted that this is the first positive case of COVID-19 among the TTC's more than 14,000 employees, and that approximately 130-170 more staffers have been "impacted by a self isolation and monitoring directive" as a result of the diagnosis.

TTC CEO Rick Leary wrote in a letter to staff early Thursday morning that the self isolation directive was enacted "out of an abundance of caution."

"The employee worked one shift on March 11 after returning from travel and was sent home sick," said Leary. "They have not returned to work since."

TTC officials say they learned of the coronavirus diagnosis late Wednesday and have since turned their attention to thoroughly disinfecting Duncan Shop "with special attention paid to common high-touch areas."

"This is an unsettling, unprecedented time at the TTC, but I want to assure you we are doing everything we can to fight the spread of COVID-19 and keep you and our customers safe," said Leary in his email to employees.

"For now, I must thank you again for all your hard work in keeping Toronto moving, even in this difficult time. I also ask you to keep this employee and their family in your thoughts."

2/2 the employee worked one shift last week at Duncan Shop where we do bus rebuilds/overhauls.
This note was sent to employees from CEO Rick Leary this morning. pic.twitter.com/gd9ANJLF7Y

— TTCStuart (@TTCStuart) March 19, 2020

Just yesterday, the TTC announced that it would allow employees to wear masks on the job after initially forbidding the practice due to uniform policy.

TTC employees who choose to wear masks may now do so for their own comfort, though the agency noted in its policy change announcement that Toronto Public Health continues to advise against mask-wearing for healthy individuals, as masks can prompt people to touch their faces more often than necessary.

As of Thursday morning, TPH was reporting 102 active cases of COVID-19 within the city, at least 11 of them potentially caused by local transmission.

"This increase of COVID-19 cases locally and within Canada is why you have seen all three levels of government implement unprecedented public health measures to require social distancing," said Toronto Medical Officer of Health Dr. Eileen de Villa on Tuesday after Ontario declared a province-wide state of emergency in response to the outbreak. 

"Social distancing is an effective way to slow infectious disease transmission. However, we won't see the effects immediately and we also know that we should expect to see continued increases in our case counts."