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.