New cases cropped up in Hong Kong, Taiwan and Orange County, Calif., bringing to five the number of confirmed cases in the United States. The virus had already been found in Thailand, France, Japan, South Korea, Australia and beyond.

Epidemiologists at Imperial College London estimated that each case infected an average of between 1.5 and 3.5 other people in the early stages of the crisis. For seasonal flu, it’s about 1.3.

That number could drop as the authorities take more stringent measures to halt the spread. But if it holds up, the number of infected could rise sharply.

In Wuhan, the city at the center of the outbreak, the streets were eerily quiet as the authorities had ordered people not to drive, forcing some to walk to hospitals. Mr. Zhou, the mayor, said that health officials were likely to confirm an additional 1,000 cases of the illness in the city. He said that the estimate was based on the assumption that around half of the city’s nearly 3,000 suspected cases of the coronavirus would eventually test positive.

China’s top leader, Xi Jinping, has promised drastic measures to contain the virus.

In a signal of the gravity of the crisis, and its likely disruption to China’s short-term growth, the government announced on Monday that the annual weeklong Lunar New Year Holiday would be extended. For now, at least, many workers will get another three days off, and go back to work on Feb. 3.

Even before that notice, Suzhou, a big manufacturing hub in eastern China, declared that factories there should not start back at work any earlier than Feb. 8.

The national government on Sunday also banned the wildlife trade until the epidemic passes. The outbreak had drawn fresh attention to China’s animal markets, where the sale of exotic creatures has been linked to epidemiological risks.