Wall Street plunged at Monday’s opening bell after a spike in the number of reported cases of coronavirus fueled fears that the epidemic would have a serious impact on global economic growth.

The Dow Jones Industrial Average fell by more than 950 points at the open, erasing all gains for the blue-chip index for the year. The S&P 500 and Nasdaq were both down by around 4 percent, marking the biggest daily drop since August.

Travel-related stocks continued to take heavy hits as the epidemic restricted movement and discouraged vacationers, with Delta Air Lines and American Airlines falling by 5 percent. Casino operators Wynn Resorts and MGM Resorts each tumbled by around 4 percent.

Scientists say the new virus, dubbed COVID-19, is both more easily transmitted and less deadly than the SARS epidemic, but much still remains unknown. As a result, said Ian Shepherdson, chief economist at Pantheon Macroeconomics, “Markets are now slaves to the news flow.”

The rate at which the virus was spreading in China appears to be slowing. An announcement of 409 new cases on Monday was the fifth day in a row that the number of new daily cases had fallen below 1,000. Outside of China, though, a spate of new outbreaks presented fresh cause for concern.

“The spike in infections in South Korea, mostly concentrated in the congregation of a single church, a surge in cases in Italy, and news of an outbreak in Iran, where the healthcare system is of uncertain quality and the government is secretive, has triggered fears that China’s aggressive quarantining efforts won’t keep the virus from spreading globally,” Shepherdson wrote in a client note.

“Global growth is likely to be impacted in a meaningful way due to fears of the coronavirus,” said Chris Zaccarelli, chief investment officer for Independent Advisor Alliance.

The challenge investors face is that no one knows how long this epidemic will last, or how dangerous it ultimately will be to populations.

“Stock markets around the world are beginning to price in what bond markets have been telling us for weeks,” Zaccarelli said. “Bond yields have continued to move lower, despite the fact that stocks quickly shrugged off the coronavirus risks last month,” he said, adding that this indicated that the stock market’s initial resilience was unlikely to last.

Nigel Green, CEO and founder of deVere Group, predicted that markets could fall by as much as 10 percent — a possibility he said most investors are not yet pricing into valuations. “Many investors remain complacent about the far-reaching impact of coronavirus, which is continuing to spread — and a faster pace. This will inevitably hit financial markets,” he said. “In general terms, stocks have hardly been deterred by the coronavirus outbreak. This complacency is concerning.”

Green added that the virus was emerging at a time when many key global economies were already vulnerable. “Coronavirus has struck at a time when major economies, including Japan, Germany, India and Hong Kong are facing a downturn due to other factors such as the U.S.-China trade dispute and political protesters, which could hit the world economy,” he said.

“This threat to global growth is real and should not be ignored,” Zaccarelli said, but he added that — provided the United State stays out of recession — the virus impact will be unlikely to hurt the retirement goals of long-term investors.

“We will move past this challenge, and the economic expansion will continue in 2020,” he said.