Ontario health officials reported 18 new cases of novel coronavirus infection in the province, the highest daily increase since the outbreak began, bringing the total number of cases including recoveries to 60.

The newest case involves Sophie Gregoire Trudeau, the wife of Prime Minister Justin Trudeau, who tested positive for the virus, the Prime Minister's Office announced late Thursday evening.

Trudeau reported flu-like symptoms on Wednesday, a week after returning from a trip in the United Kingdom. She attended an event in London with her daughter and the prime minister's mother.

The cases originate across the province, with two in Waterloo Region, one in Barrie, nine in Toronto, one in Ottawa, one in Hamilton, two in Peel Region and one in Halton Region.

They range in age from a boy under the age one-years-old who reported to North York General Hospital to a man in his 70s who went to hospital in Mississauga with symptoms after travelling to the United States.

The baby boy is considered a “close contact” of a previously confirmed case.

Eight of the new cases reported recent travel to the United States or its territory of Puerto Rico, One case recently returned from Italy, another returned from Spain and another returned from a myriad of travel across Europe.

Two others, a woman in her 20s in Peel Region and a man in his 50s in Hamilton reported travel but the locations are “pending” and still under investigation.

Four of the cases are considered close contacts of previous cases.

Toronto Chief Medical Officer of Health Dr. Eileen de Villa said the city now has 29 active cases, and no evidence yet of community spread.

“At this time the virus is not circulating locally, however given the global circumstances, Toronto Public Health is actively working with City and health partners to plan for the potential of local spread,” de Villa said.

All seventeen new patients were discharged from various hospitals and put into self-isolation.

Thursday’s announcement grew the number of active cases in Ontario by 46 per cent over Wednesday’s total, and all the travel related cases will require additional efforts by public health officials in terms of contact tracing over a period of days.

Region of Waterloo Public Health issued additional information about their two new cases.

They say one of their patients, a woman in her 60s, returned to Canada on March 8 after embarking on a Celebrity Cruise that landed on the islands of St. Thomas, St. Martin, Barbados, Dominique, Grenada and Puerto Rico.

On March 10, she drove to Kitchener’s Grand River Hospital for testing as she reported “mild respiratory symptoms.”

Their other patient is a man in his 40s who recently returned from a trip to Las Vegas.

On March 10 he also drove to Grand River Hospital after developing symptoms.

Officials say they are working to trace the man’s close contacts since his return.

The increase in COVID-19 cases prompted health officials to advise anyone returning from travel abroad to avoid public gatherings and not to visit vulnerable populations such as seniors.

"If you have gatherings of over 1,000, I would prefer that you did not hold those events, that you would postpone or delay those," Dr. David Williams said. "If you have gatherings of between 250 and 1,000 … consult with your local public health."

De Villa said that while there is no "confirmed evidence of local transmission" of the virus at this point, officials do expect that to "eventually occur."

In the meantime, she said that residents need to change some of their behaviour, such as avoiding hand-to-hand contact through the shaking of hands.

Meanwhile, Prime Minister Justin Trudeau and his wife Sophie Gregoire-Trudeau are in self-isolation over COVID-19 concerns, which has forced the cancellation of an in-person meeting of Canada's first ministers.

A further 536 people are under investigation for possible coronavirus infection, up from 465 yesterday.

Last month, provincial officials said the Public Health Ontario lab in Toronto can complete 1,000 COVID-19 tests per day.

Canada now has 158 cases of COVID-19 including recoveries, as well as one fatality.