The woman in her 60s returned from China on January 13 without showing any signs of illness, but three or four days later she called her doctor to report feeling sick.

The patient is doing well and remains hospitalised "primarily for infection control," said Dr. Allison Arwady, Chicago's public health commissioner.

Earlier this week, a man in Washington state became the first US patient, also diagnosed after returning from a trip to the outbreak's epicentre in central China.

The Centers for Disease Control and Prevention is expecting more Americans to be diagnosed with the newly discovered virus in coming days, as worldwide the number of confirmed cases has passed 800.

The virus can cause fever, coughing, wheezing and pneumonia. It is a member of the coronavirus family that's a close cousin to the deadly SARS and MERS viruses that have caused outbreaks in the past.

"CDC believes that the immediate risk to the American public continues to be low at this time, but the situation continues to evolve rapidly," CDC's Dr. Nancy Messonnier said.

The Washington state patient was in satisfactory condition Friday in an isolation unit at Providence Regional Medical Center in Everett, 30 miles north of Seattle.

Nurses who've trained for Ebola patients are providing his care, wearing hoods with plastic face shields and using respirators to breathe filtered air.

They are bagging and storing disposable gloves, linens and gowns until the CDC tells them what to do with the patient's medical waste.

Nationally, over 2000 returning travellers had been screened at US airports and 63 patients in 22 states were being tested, although 11 of them so far have been found free of the virus, the CDC said.

Australian Associated Press