The major stock indices sank lower this week as U.S. health officials confirmed the nation’s first person-to-person transmission of the coronavirus Thursday, which has already taken the lives of at least 171 people in China and infected more than 8,200 since it was revealed less than a month ago.

The most recent patient is the husband of the Chicago woman who carried the infection back from Wuhan, China, the epicenter of the outbreak, CDC and Illinois health officials explained during a press briefing. Health officials said the man, in his 60s, has “some underlying medical conditions” but was in good condition. His wife was doing well but remained in isolation at a local hospital, they said.

Now public health officials are also monitoring 21 patients in Illinois for possible infections.

“This is a very serious public health situation,” said Dr. Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases. “Moving forward, we can expect to see more cases, and more cases means more potential for person-to-person spread.”

Although there has been much speculation as to how the virus is spread, health officials said Thursday that they still are unsure whether the virus is infectious before symptoms. Officials are working with hospitals to ensure that health-care providers are “protected,” Messonnier said.

“Despite the case that we are reporting the first instance of person-to-person transmission in the United States, it is important to note that these two individuals were in close contact,” she added.

The coronavirus has already spread to a cluster of people via human-to-human contact outside of China, Dr. Mike Ryan, executive director of the WHO’s health emergencies program, said at a news conference at the organization’s Geneva headquarters Wednesday.

“These developments in terms of the evolution of the outbreak and further development of transmission, these are of grave concern and has spurred countries into action,” Ryan said. “What we know at this stage, this is still obviously a very active outbreak and information is being updated and changing by the hour.”

Since the markets first reacted to the rapid spread of the virus, volatility has expanded rapidly, as the S&P 500 has lost nearly 100 points, or 3% from its recent high, with the other indices following suit.

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