TORONTO — A fourth person in Canada has tested positive for the novel coronavirus, Canadian public health officials said Friday.

The woman, who is a student in her 20s at Western University in London, Ontario, returned from a trip to Wuhan on Jan. 23. Her parents were sick, but their illnesses were thought to be “mild,” so they weren’t tested for the virus at a hospital there, said Chris Mackie, the medical officer of health for the Middlesex-London Health Unit.

The woman was not symptomatic on her flight home and wore the “equivalent of a surgical mask” out of an abundance of caution, said Mackie. She felt unwell the following day and went to London’s University Hospital. She was tested for the virus and is in isolation at home, where she is “perfectly well now,” Mackie said. She has not been out in public since returning from China and recovered after three days, he added.

Tests for the coronavirus run in Ontario came back negative, but some of the five tests run at the National Microbiology Laboratory in Manitoba came back as “weakly positive,” with low levels of virus in the sample, said David Williams, the chief medical officer of health for Ontario. She was then switched from a negative case to a positive one.

Officials would not provide any details about the woman’s flight because she was not symptomatic while traveling and they continue to assert that the virus cannot be transmitted if a person is not experiencing symptoms.

The patient with Canada’s first confirmed case of the novel coronavirus was discharged from Toronto’s Sunnybrook Health Sciences Center on Friday and will remain in self-isolation at home with his wife, who is also suffering from the virus.