Stocks in the United States suffered their worst decline since early October, amid concern that a dangerous new virus could hurt the global economy.

On Wall Street, the S&P 500 and the Nasdaq composite index both dropped 0.9 percent on Friday. The declines picked up steam after officials said a second case of the virus, the coronavirus, had been diagnosed in the United States. The patient, a woman in her 60s in Chicago, had recently traveled to Wuhan, China, the center of the outbreak.

Earlier Friday, the Chinese authorities broadened a travel lockdown in central China to 12 cities near Wuhan. The orders effectively pen in 35 million residents to try to contain the virus, which had led to at least 41 deaths by Friday evening and more than 1,000 infections worldwide.

Airline shares — exposed to the impact of travel restrictions related to the virus — dropped sharply, American Airlines by 4 percent and United Airlines by 3.5 percent.