McLaren pulled out of the season-opening Australian Grand Prix Thursday after a team member tested positive for coronavirus, throwing the race into chaos.
The employee was among eight Formula One personnel who went into isolation after showing flu-like symptoms typical of the virus this week in Melbourne.
The other seven -- including four from the Haas team -- all returned negative results, organisers said.
"The team member was tested and self-isolated as soon as they started to show symptoms and will now be treated by local healthcare authorities," McLaren said in a statement.
"The decision (to withdraw) has been taken based on a duty of care not only to McLaren F1 employees and partners, but also to the team's competitors, Formula 1 fans and wider F1 stakeholders."
McLaren's decision to pull out casts doubt over whether the race will be run without a full complement of teams, and whether others will follow suit.
The Australian Grand Prix Corporation said it was "currently in discussions with Formula 1, the FIA and the (Victorian) department of health and human services in relation to the broader implications of this test result".
World champion Lewis Hamilton earlier Thursday said he was stunned the race was going ahead as fears mount about the spread of the disease.
"I am really very, very surprised that we're here. I don't think it's great that we have races but it really is shocking that we're all sitting in this room," he said at an official pre-race press conference packed with media.
"It seems that the rest of the world is already reacting a little bit late, but you have seen this morning with (President Donald) Trump shutting down the border to Europe to the States, the NBA suspended, yet Formula One continues to go on.
"It's a concern I think for the people here. It's quite a big circus that's come here, it's definitely concerning for me."
- 'Cash is king' -
The Mercedes star, who is gunning to match Michael Schumacher's record seven world crowns this season, sat alongside Ferrari's Sebastian Vettel and Renault's Daniel Ricciardo in the media session, but a large open space separated them from the press.
Asked why he felt the race was still on, Hamilton replied: "Cash is king."
Four-time world champion Vettel said it was difficult to judge what to do as the virus continues its spread.
"Obviously we have to trust the FIA (International Automobile Federation) to take precautions as much as they can, but I think the answer that nobody can give you at the moment is how much you can control what is going on," he said.
"As a matter of fact, we are here so you just try to take care as much as you can."
Australia has reported 150 cases of coronavirus so far, including among fans who attended the women's T20 Cricket World Cup final and a Super Rugby match, both in Melbourne last week.
European countries that are home to many of the F1 teams and journalists have had far more cases.
Despite concerns, fans flocked to Albert Park on Thursday for a Supercars qualifying session.
"I'm not worried, I'm washing my hands and that's the best thing to do," said spectator Robert Clarke as he used a hand-sanitiser station.
The first F1 practice sessions are due to start on Friday.
In an attempt to limit interaction between drivers and fans, autograph sessions have been replaced by question and answer interviews, with selfies banned.
Media events have also been hit with Renault's Ricciardo and Esteban Ocon "excused" from a press conference Wednesday, and an exclusion zone was enforced around Max Verstappen and Alex Albon at a Red Bull function.
Ocon was spotted wearing a mask in the paddock on Thursday, while teams scrapped all-in TV interviews, where media are tightly packed around the drivers, for the duration of the weekend.
The coronavirus has already hurt the sport with April's Chinese Grand Prix postponed, while the second race of the year in Bahrain will be held without spectators.
