A fourth person in Canada has the coronavirus, first testing negative, then positive before recovering quickly as health officials scramble to get a handle on the fast-moving infection.

The female university student in her 20s arrived in Toronto on Jan. 23 from Wuhan, China, the epicenter of the outbreak. Initially asymptomatic, she reported to hospital on the 24th where she initially tested negative. A second test at Canada’s National Microbiology Laboratory in Winnipeg, Manitoba, confirmed the case as positive.

“It is clear that we are learning more and more about the coronavirus each day, and our testing procedures are evolving and getting more and more precise,” Ontario’s Chief Medical Officer of Health Dr. David Williams said in a statement after a news conference in Toronto.

Health officials globally are struggling to understand the nature of the disease and how it is transmitted, as well as how to diagnose it. There are reports that it can be passed by people before they show symptoms but not all health officials believe this is the case.

Risk “Low”

There is still no “hard evidence” that a person without symptoms is infectious, Barbara Yaffe, associate chief medical officer of health, Toronto said at the news conference.

The female patient’s viral load was extremely low and she fully recovered in two to three days but remains in isolation, officials said.

“The risk continues to be very low for Ontarians,” Williams added at the news conference.

The new Ontario case brings the number of infected in Canada to four, with one in British Columbia.

The first confirmed instance of the virus was announced Jan. 25. In the first case, a man in his 50s, who had traveled from Wuhan via Guangzhou, checked into a Toronto hospital a day after he landed on Jan. 22.

He is still recovering, indicating recovery times aren’t the same across the board, health officials said.