CHICAGO – Health officials on Thursday reported the first U.S. case of person-to-person spread of coronavirus – the second confirmed case in Illinois and sixth in the nation.

The first case of person-to-person transmission is the husband of a Chicago woman who developed symptoms after visiting China.

The woman, who is in her 60s, was hospitalized after being diagnosed with the illness following her return from Wuhan, China, the epicenter of the virus outbreak, on Jan. 13. She and her husband, who is also in his 60s and did not join her on the trip, are hospitalized.

"This news may raise people’s concerns, but I want to state clearly that this development is something that we have been prepared for," Allison Arwady, chief medical officer at Chicago Department of Public Health, said at a press conference Thursday. "It's not totally unexpected that he acquired the virus."

"I’m not surprised by a sporadic second-generation case," said Greg Poland, director of the Mayo Clinic Vaccine Research Group. "But everybody's holding their breath to see if we’ll have third or fourth generation cases. Did this spouse spread the virus?"

In fact, at a press briefing last week, the CDC's Dr. Nancy Messonnier warned that "We are likely going to see some cases among close contacts of travelers and human-to-human transmission."

Ngozi Ezike, director of the Illinois Department of Public Health, said 21 people are being investigated for coronavirus. Health officials said that it is still not yet clear how easily the virus spreads from one person to another.

Arwady described the Chicago woman as "doing well" and said her husband is in "stable condition."

AMITA Health St. Alexius Medical Center Hoffman Estates, where the woman was being treated in isolation, is also treating the husband.

"Both patients are being monitored in isolation at our hospital in accordance with established infection control protocols and with guidance from the CDC," AMITA Health said in a statement. "We have in place strict infection control precautions and protocols established by the CDC to protect patients, associates and visitors."

Arwady said health officials had been actively monitoring and proactively testing the husband, as he had been in close contact with his wife after she had developed symptoms.

The man developed symptoms on Tuesday and was immediately placed in isolation, Arwady said. Illinois health officials received the man's positive lab result from the CDC Wednesday night.

"We know already that, just like his wife, he has not taken the L (Chicago's elevated train line). He has not attended any large gatherings," Arwady said.

Health officials are following up with anyone the husband had contact with, Arwady said.

"There is no need for the general public to change their behavior in any way based on today’s news," Arwady said. "There is no local emergency."

Arwady said Illinois residents do not need to stay home or cancel events.

Within hours of last week’s announcement that there was a confirmed case of coronavirus in Illinois, the lone Walgreens in Chicago's Chinatown neighborhood had sold out of face masks, and Lunar New Year events across the city were canceled, the Chicago Tribune reported.

Dr. Igor Koralnik, a neuro-infectious diseases specialist at Northwestern Medicine, discouraged people from wearing protective masks.

"It’s not a good idea to start wearing those masks because it can cause concern among those around you," Koralnik said. "I don’t think it’s time for a mass hysteria or panic in the U.S."

The four other U.S. cases, which occurred after the infected persons visited China, are in Arizona, Southern California and Washington state.

News of the first person-to-person coronavirus transmission comes the same day as the World Health Organization declared coronavirus a global health emergency.

Follow Grace Hauck on Twitter at @grace_hauck.