By noon Monday that all changed. Amid reports that the coronavirus had spread well beyond China and that Beijing might lower its guard and ease its quarantines, the American market suffered its worst day in two years and ended down by nearly 5 percent from its peak last week. That drop is now right in line with the average at this stage of the eight postwar contagions, which go back to the Asian flu of 1957. Markets are worried — but as the doctors would say, within normal limits. By Tuesday morning, a semblance of stability was returning to global markets.

What happens next depends in part on the impact to the global economy. The latest estimates suggest that the hit from the coronavirus could make the first three months of 2020 the slowest quarter for global growth since the crisis year of 2008.

If the past is any guide, however, the growth scare should be relatively brief. Recent global contagions going back to the SARS virus in 2003 have seen a sharp slowdown lasting about a quarter, followed by a sharp recovery over the next quarter. That’s why the consensus on Wall Street is that there will be no global recession, and within six months the whole scare will be over.

Still, the quarantine of 16 cities in China has had a visible impact on economic activity, reducing traffic on roads, railways and at airports, emptying out theaters and other public spaces. Analysts skeptical of official Chinese data have started to track alternative sources, from satellite images of road congestion in urban areas to the density of smog over Hong Kong and traffic on Chinese search engines. All those unofficial trackers also point to a hard hit.

Beyond China, however, the economic impact will vary from nation to nation, depending on how much stimulus a country can afford. Already Beijing has been cutting interest rates and encouraging banks to increase lending. Investors are confident the authorities will do whatever it takes to keep the economy moving. That is one reason, despite the fact that China is ground zero of the epidemic, that markets have held up better there than in other developing countries.