The latest Illinois case brought the tally of confirmed U.S. coronavirus infections to six, none fatal, according to the Centers for Disease and Prevention (CDC), as the number of Americans potentially exposed to the virus and placed under medical observation continued to rise.

One of 195 U.S. citizens evacuated from Wuhan, China, and voluntarily confined to a military base near Los Angeles for evaluation was slapped with a mandatory quarantine after trying to leave the facility, health officials there said on Thursday.

The quarantine order, issued Wednesday night by the county health agency, requires the individual to remain at March Air Reserve Base for the full 14-day incubation period of the virus, rather than 72 hours as prescribed by the CDC to screen evacuees before they are released.

None has shown any symptoms, a county health spokeswoman said. The quarantined person, whose identity was not disclosed, is complying with the order, she added.

The first five known U.S. patients are believed to have contracted the virus during visits to China, the epicenter of the worldwide outbreak, whereas the newly diagnosed Illinois patient - confirmed by the CDC on Wednesday - was infected by his wife.

"This is the first case of person-to-person transmission in the U.S.," said Dr. Allison Arwady, a commissioner at the Chicago Department of Public Health. Even so, Arwady added, "there is no local emergency."

Concern about the new virus has mushroomed around the world as more cases turn up outside China. Health authorities still do not know how dangerous the virus is or how easily it spreads.

The World Health Organization on Thursday declared the coronavirus outbreak in China a global emergency as cases spread to 18 countries. More than 200 deaths and nearly 10,000 cases have been reported in China, the vast majority in and around Wuhan, China, where it is believed to have originated in a market that traded in illegal wildlife.

U.S. Risk Still Seen as Low

Airlines were facing mounting pressure by cabin crew to stop all flights due to unease about exposure to the rapidly spreading coronavirus, with American Airlines' pilots filing a lawsuit seeking an immediate halt

The Illinois Department of Public Health said it was tracking 21 people who had contact with the infected Illinois couple, both in their 60s. Officials said the husband had not recently taken public transportation or attended any large gatherings.

He has been in isolation the past two days after showing symptoms and was immediately hospitalized, Arwady said.

CDC experts said they were not surprised by the development and that further person-to-person cases were likely to emerge.

"We understand that this may be concerning, but based on what we know now, we still believe the immediate risk to the American public is low,” CDC Director Dr. Robert Redfield said in a release on Thursday.

Following the airlift of State Department employees and other Americans from Wuhan, Washington also was evacuating non-emergency government employees and family members from the U.S. Embassy in Beijing and consulates in Chengdu, Guangzhou, Shanghai and Shenyang due to the coronavirus outbreak, a State Department official said on Thursday.

The department also will charter additional evacuation flights from Wuhan, leaving on Monday or sooner, it said in a statement on its website.

REUTERS