The Centers for Disease Control and Prevention and Illinois public health officials confirmed Thursday the nation’s first person-to-person transmission of the coronavirus that has already killed at least 171 people in China and infected more than 8,200 since emerging less than a month ago.

The new patient is the spouse of the Chicago woman who brought the infection back from Wuhan, China, the epicenter of the outbreak, CDC and Illinois health officials said during a press briefing. Health officials said the new patient, a male in his 60s, has "some underlying medical conditions" but is currently in good condition. The first patient is doing well but remains in isolation at a local hospital, they said.

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."

The virus, which emerged Dec. 31, has already spread to more people than the 2003 SARS epidemic that sickened roughly 8,100 people across the globe over nine months. The transmission makes the U.S. at least the fifth country where the infection is now spreading through human-to-human contact, including China. CDC officials said there are at least nine cases of human-to-human transmission outside of China, as of Thursday.

As of Thursday morning, 8,130 cases were confirmed in mainland China alone, according to Chinese state media, and more than 100 cases were confirmed elsewhere around the world — bringing the global total to at least 8,240.

U.S. officials are working to contain the outbreak from spreading in the country, including by increasing travel warnings and expanding screening at 20 U.S. airports. Trump administration officials have said they are constantly evaluating the situation and could take more drastic action if needed.

Health officials said Thursday that they still don’t know whether the virus is infectious before symptoms. Officials are working with hospitals to make sure 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 World Health Organization is reconvening an emergency meeting Thursday at 2:30 p.m. ET where the organization will decide whether the new virus constitutes a global health emergency.

The coronavirus has spread to a handful of people through 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."

WHO Director-General Tedros Adhanom Ghebreyesus said the "continued increase in cases and the evidence of human-to-human transmission outside of China are, of course, most deeply disturbing." The illness produces a range of symptoms, with about 20% of the patients developing severe illnesses, including pneumonia and respiratory failure, he said.

"Although the numbers outside China are still relatively small, they hold the potential for a much larger outbreak," Tedros said.

CDC officials said Wednesday that a flight carrying U.S. citizens who evacuated from Wuhan, the epicenter of the coronavirus outbreak, safely arrived in California. Officials said all passengers were asked to remain at the March Air Reserve Base for further CDC screening and testing.

As of Wednesday, the CDC was monitoring 165 people across 36 states for the virus, including five patients who contracted the deadly infection in China and brought it back to the U.S. and one of the spouses. At least 68 people have tested negative for the virus, and the CDC is awaiting results for 92 individuals. World health officials say the respiratory disease spreads through human-to-human contact, droplets carried through sneezing and coughing and germs left on inanimate objects.