“We’d absolutely expect to see a response from the Federal Reserve, not least to shore up confidence,” said Paul Ashworth, an economist at Capital Economics, a research consultancy. But he pointed out that monetary policy worked on the economy with a six- to nine-month lag, and “it doesn’t deal with the supply-side impact of, say, one-third of your work force catching this.”

The more critical response may come from Congress and the Trump administration, which have done little thus far to script a fiscal response.

Perhaps the most important thing the government can do to insulate the economy is to stem the outbreak, keeping Americans on the job and spending. If that fails, though, fiscal responses are an option; Hong Kong and China, both hit hard, have rolled out packages to help bolster growth. Tax and spending policies might also encourage demand more than fixing supply, but they can also work more quickly than monetary policy.

House Speaker Nancy Pelosi of California and Senator Chuck Schumer of New York, the Democratic leader, on Thursday morning called for Congress and Mr. Trump to fashion a spending bill meant to “address the spread of the deadly coronavirus in a smart, strategic and serious way.” A response should include interest-free loans for “small businesses impacted by the outbreak.”

Such a program would represent targeted relief but not an effort to dramatically increase consumer demand in the economy.

But such a plan seems far-off, if not improbable. Democratic and Republican leaders in Congress have not opened talks with the White House or between the House and Senate over any possible package of tax cuts and spending increases that would be meant to stimulate the economy in the event of a virus-related downturn. Top Senate aides said on Thursday that it was too soon for such conversations, with Mr. Trump’s allies noting the persistence of low unemployment and continued economic growth.

Michael Zona, a spokesman for the Senate Finance Committee and its chairman, Charles E. Grassley of Iowa, said on Thursday that “at this point, the coronavirus has not had a broad impact on the U.S. economy, and its effects have been limited.” But Mr. Zona said Mr. Grassley and the committee were “ready to consider appropriate tax relief responses if that becomes necessary and the extent of the problem can be determined.”