Stocks fell on Wall Street in midday trading Thursday as investors worried that a deadly virus outbreak in China could continue spreading and hurt the global economy.

The slump for U.S. indexes follows a sell-off for global markets as authorities worldwide take measures to monitor and contain the virus. The central Chinese city of Wuhan, where the virus is concentrated, closed down its train station and airport Thursday to prevent people from entering or leaving the city.

The coronavirus has been confirmed in five countries, including China, the U.S., Thailand, Japan and South Korea. More than 500 people have fallen sick and 17 have died from the illness, which can cause pneumonia and other severe respiratory symptoms. A World Health Organization committee was scheduled to meet for a second day Thursday as it decides whether to declare China’s virus outbreak a global health emergency.