Live Coronavirus Updates and Coverage Globally

In the United States, the sense of being cut off increasingly seems to apply to the states themselves, as Washington has been slow to produce either promised aid or a coherent strategy and President Trump advised governors — some of whom were shocked — that they should look to buy their own ventilators and respirators, which are in desperately short supply.

On Monday, health officials ordered millions in six counties in the Bay Area to “shelter in place,” one of the most significant restrictions yet to American life in the race to stop the coronavirus outbreak from surging in the United States.

The order, which goes into effect Tuesday, is expected to disrupt life for millions of residents in Alameda, Contra Costa, Marin, San Francisco, San Mateo, and Santa Clara Counties. The City of Berkeley also issued the same order.

And families were left to worry about lost wages, about inadequate supplies of medicine and protective gear, about leaving the ill and elderly even more isolated and vulnerable, and about jobs, institutions, relatives and neighbors that might vanish and never return.

Scientists tracking the spread of the coronavirus reported Monday that, for every confirmed case, there are most likely another five to 10 people in the community with undetected infections. These often-milder cases are, on average, about half as infectious as confirmed ones, but responsible for nearly 80 percent of new cases, according to the report, which was based on data from China.

The researchers modeled the virus’s natural spread in China before the government instituted a travel ban and an aggressive testing policy. During that time, from December of last year through late January, about 6 in 7 cases went undetected. That situation is analogous to the current state of affairs in the United States and other Western countries, where tests are not widely available, the researchers said.

“If we have 3,500 confirmed cases in the U.S., you might be looking at 35,000 in reality,” said Jeffrey Shaman, an epidemiologist at Columbia University and the senior author of the new report, which was posted by the journal Science.