The increase, in part, prompted the CDC to mandate that the U.S. citizens who were flown in from Wuhan this week stay on March Air Reserve Base in California for a total of 14 days since they left China. Previously, the passengers had been asked to stay on base for 72 hours, pending test results for the new coronavirus.
The incubation period, or the time it takes for a person to become ill after being exposed to this coronavirus, is believed to be no more than 14 days, the CDC said.
But on Friday, Messonnier said that until more is known, an early negative test for the new coronavirus does not guarantee a person won't get sick.
"We would rather be remembered for overreacting than underreacting," she added. "While we recognize this is an unprecedented action, we are facing an unprecedented health threat."
The epicenter of the outbreak remains in China, where more than 200 people have died. On Thursday, the World Health Organization declared the situation a global public health emergency.
While coronaviruses in general are well-documented (they're responsible for some forms of the common cold, for example), the new coronavirus called 2019-nCoV was first detected in humans just last month.
That means little is known about its spread, fatality rate or when patients are most contagious. Scientists worldwide are learning about the virus and the illness it causes in real time.
Full coverage of the coronavirus outbreak
Six cases have been diagnosed in the United States, including one in Washington, one in Arizona and two in California.
Two cases have also been diagnosed in Chicago, representing the first instance of a person-to-person spread of the virus in the U.S. The patients are a married couple. The wife had traveled to Wuhan in December to care for her elderly father before returning to her home in Chicago on Jan. 13, apparently feeling well. She was later diagnosed with the new coronavirus and hospitalized in isolation.
Her husband, who had not traveled to China with her, later became ill and is now also confirmed to have the virus. Both remain hospitalized in good condition.
Also, Thursday night, the New England Journal of Medicine published a letter from doctors in Germany who documented a case of a coronavirus patient infecting a colleague before she'd developed symptoms — a phenomenon known as asymptomatic spread.
But even if it is confirmed that the virus can spread before symptoms show up, health officials caution that it's not likely to make the coronavirus more dangerous.
"The driver of outbreaks is always a symptomatic person," Dr. Anthony Fauci, director of the National Institutes of Health's National Institute of Allergy and Infectious Diseases, said during a news conference this week.
"In all the history of respiratory-borne viruses of any type, asymptomatic transmission has never been the driver of outbreaks," he added.
On Friday, Messonnier also cautioned against stigmatizing the estimated 4 million Chinese Americans in this country.
"Do not assume that just because someone is of Asian descent, that they have this new coronavirus," Messonnier added. "Do not let fear or panic guide your actions."
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