That count, which involves brief human contact on a broad scale, could resume when the suspension ends on April 2 if bureau officials are satisfied that workers can be adequately protected, Ms. Lowenthal said.

The second count is of roughly 500,000 homeless people in shelters, temporary housing and on the street, scheduled for two days beginning March 30. Medical experts have singled out the homeless as especially vulnerable to picking up and spreading the coronavirus because of their living conditions and poor health. Experts said the bureau has postponed that count while it tries to devise safer ways for enumerators to conduct the tally.

Most training of census workers is conducted online, but experts said the bureau is also suspending in-person training and intends to move those exercises online as well.

Steve Jost, a former spokesman for the Census Bureau, said in an email that a delay “greatly increases the risk to the accuracy and completeness of the count, and possibly the delivery of the data products.” The activities affected by the suspension are labor-intensive and “essential to a complete count,” he said.

What happens to the rest of the census’s sprawling nationwide apparatus was not clear. A third in-person count targeting remote areas like mountain cabins and lake cottages with no fixed address appears so far to be unaffected. Nor did the bureau’s announcement discuss safety measures at dozens of regional headquarters and two data centers it operates, in Tucson, Ariz., and Jeffersonville, Ind., that employ large numbers of seasonal workers.

And the statement only briefly mentioned perhaps the biggest challenge of all: finding a way to shield the hundreds of thousands of enumerators who will be deployed in late May to knock on the doors of every household that did not respond to an invitation to fill out census forms.

Their work used to entail long sessions in living rooms and kitchens where residents filled out paper forms. This year, those census-takers should be able to conduct most of their work from front porches, asking questions and punching the results into iPhones that transmit the answers to remote computers.