What to Know At least 21 people were under investigation for potential coronavirus exposure in Illinois as of Thursday, state health officials said

Two people in Illinois are among the six confirmed case in the U.S.

The latest Chicago case marks the first case of person-to-person spread in the U.S.

The infant grandson of a Chicago man who was diagnosed with coronavirus has developed a fever and is being tested for the virus, family members told NBC News.

The baby and other family members who had close contact with the man were under isolation following the recent diagnosis. The infant was taken to an area hospital after his fever developed, though it remains unclear if the child was experiencing any other symptoms and the cause of the fever remained unclear.

At least 21 people were under investigation for potential coronavirus exposure in Illinois as of Thursday, state health officials said.

The child's grandfather this week became the second person to test positive in the state and the first case of human-to-human transmission in the U.S., health officials announced. The man's wife, who had traveled to Wuhan, China - the epicenter of the recent outbreak - was diagnosed with coronavirus last week.

"This is a rapidly evolving situation and the status of individuals will change as we investigate," said Dr. Ngozi O. Ezike, director of the Illinois Department of Public Health.

A relative of the Chicago woman who was diagnosed with coronavirus last week has now tested positive for the virus, marking the first instance of person-to-person spread in the U.S., the Centers for Disease Control and Prevention announced Thursday. Listen to the CDC's announcement.

The pair had been in "close contact" before the virus spread, health officials said. Both were hospitalized in Hoffman Estates.

"This may be concerning, but based on what we know now, our assessment remains that the immediate risk to the American public is low," said Robert Redfield with the CDC.

The woman was diagnosed with the virus after she returned to the U.S. earlier this month and started experiencing symptoms including a fever and shortness of breath.

Health officials urge caution after a person in Indiana may have been infected with coronavirus. NBC 5's Patrick Fazio has the details on what is known so far.

She had "very limited movement outside her home" following her return from China, public health officials said, and did not take public transit or attend any large gatherings.

"Again this is not spreading widely in communities and the general public is believed to be at low risk," said Jennifer Layden, Illinois state epidemiologist.

Still, DuPage County health officials said Wednesday they were contacting residents who may have been exposed to the Chicago woman, saying that some residents "meet the definition of persons under investigation and who are currently being tested."

DuPage County health officials discuss how they are contacting residents who may have been exposed to an Illinois patient diagnosed with the coronavirus last week.

"It can be a point of concern but we are not considering it to be a point of panic or alarm," said DuPage County Health Department Executive Director Karen Ayala, adding that the "general population is at low risk."

The department declined to give numbers on how many people were contacted but said members of the community "have had some sort of contact" with the Chicago woman.

"In a county of a million, it’s a very small amount of folks," Ayala said.

While there are many questions still left to be answered on the new virus, such as how quickly it spreads, the CDC said it expected some person-to-person cases to develop.

The latest diagnosis brings the total number of cases in the US to six, though the five other patients had each traveled to the hardest-hit part of China before becoming ill.

The Indiana State Department of Health and the Centers of Disease and Control issued a statement Tuesday indicating that they have identified a person who could potentially be infected with the novel coronavirus.

"Final diagnosis awaits the results of laboratory tests to be performed by the CDC," the statement read. "Out of an abundance of caution, the individual has been placed in isolation and is currently under active medical supervision.”

The virus has already killed more than 200 people and infected nearly 10,000 across the globe, sparking travel warnings and airport screenings in multiple countries.

British Airways has halted all flights to China and American Airlines stopped flights out of Los Angeles to and from Shanghai and Beijing. Delta Airlines announced Friday it has decided to temporarily suspend all U.S. flights to China flying beginning next week — and lasting through April — due to ongoing concerns about the coronavirus.

Citizens from the US and European countries have been evacuated from central China due to concerns over the virus.