The first known coronavirus case in the United States involving someone who had not traveled to China has surfaced in Illinois, health officials said Thursday.

The patient, who was not identified, is the husband of a Chicago woman who got sick after returning from a trip to Wuhan, the epicenter of the outbreak, according to the Centers for Disease Control and Prevention.

The man, who is in his 60s, began feeling ill Tuesday and was promptly placed in isolation, officials said.

There are 21 people, including health-care workers, who may have had close contact with the patient and are being monitored.

“The risk to the general public remains low,” said Dr. Ngozi Ezike, director of the Illinois Department of Public Health.

Still, officials warned that the case of human-to-human transmission indicated that the coronavirus was a “very serious public health situation.”

“Moving forward, we can expect to see more cases, and more cases means more potential for person-to-person spread,” said Dr. Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases.

The report marked the sixth coronavirus case, including the man’s wife, that has been detected in the US.