A cruise ship carrying 3,500 people is waiting off the coast of California while tests for coronavirus are carried out on dozens of them.
One passenger died and at least four others became infected on a previous voyage by the ship, the Grand Princess.
Test kits were delivered by military helicopter on Thursday as the vessel sat off San Francisco.
The US has now recorded about 200 cases and 12 deaths as the global struggle with the virus continues.
More than 92,000 cases have been recorded worldwide, with 80,552 in China, where the virus originated. China also accounts for the vast majority of deaths – 3,042 to date.
Stocks in Asia and the UK fell sharply on Friday as investors continued to worry about the broader economic effects of the virus.
Why is the cruise ship under such scrutiny?
Californian Governor Gavin Newsom has refused to allow the ship to dock until its passengers are properly assessed. Test results are expected later on Friday.
There are 142 Britons among those aboard the ship, which is returning from a voyage to Hawaii after a trip to Mexico last month.
A man who sailed on the Grand Princess to Mexico died of coronavirus on Wednesday. The 71-year-old from the Sacramento area had underlying health conditions, officials say.
Two other passengers from that voyage are in hospital with the virus in northern California while two Canadians who recently sailed on the ship tested positive after returning home.
Another US passenger from the Mexico voyage, Judy Cadiz, said she and her husband had fallen ill after returning but had not given it much thought until learning of the death.
Now they cannot get a straight answer about how to get tested, she was quoted as saying by the Associated Press.
“They’re telling us to stay home, but nobody told me until yesterday to stay home,” she added.
“We were in Sacramento, we were in Martinez, we were in Oakland. We took a train home from the cruise. I really hope that we’re negative so nobody got infected.”
Another cruise ship, the Diamond Princess, saw a dramatic outbreak while being held in quarantine in Japan last month. Six people died, with 621 infected.
Both ships are operated by the same company, Princess Cruises.
The US Congress is rushing to approve a bumper emergency aid package to combat coronavirus, with the White House acknowledging the country lacks enough test kits.
Nine of the 12 fatalities were from the same nursing home in Seattle, Washington State, but infections have been reported across 20 states.