One of the final sentences of the Phase 1 trade deal may prove to be key. The provision calls for consultations between the parties if “a natural disaster or other unforeseeable event outside the control of the Parties delays a Party from timely complying with its obligations under this Agreement.”

But even with a deadly disaster looming, China’s failure to meet its commitments may create some opposition in the United States, potentially returning the countries to their rockier relations before the signing of the trade deal.

Economists have predicted a drag on global growth from the virus, at least in the short term. In the United States, Goldman Sachs analysts estimate a 0.4 percentage-point reduction in first-quarter economic growth, though that effect is likely to fade.

Those costs could quickly outweigh the economic benefits of the trade agreement. While the Trump administration has touted big economic gains from the pact, economists’ forecasts have been modest, since the deal leaves tariffs in place on more than $360 billion of Chinese goods.

Privately, some Trump administration officials say that China may use the virus as an excuse to delay meeting its commitments, in hopes that Mr. Trump will ultimately be voted out of office this year.

Some in China have reacted negatively to the Trump administration’s decision to restrict travel between the countries, including barring entry to all foreign nationals who recently traveled in China.

In a note to clients, Ian Bremmer, the president of Eurasia Group, said that the Chinese government had found the American move to shut its borders “unnecessarily provocative, and it adds a sour tone on the back of the recently agreed Phase 1 trade deal,” he said.