(Reuters) - The U.S. Centers for Disease Control and Prevention (CDC) on Friday confirmed a second U.S. case of the new coronavirus from China in a Chicago woman, and said as many as 63 potential cases were being investigated as the sometimes deadly illness continues to spread around the globe.

Of the 63 people under investigation from 22 states, 11 have so far tested negative, CDC said on a conference call with reporters.

News of the woman in Chicago infected with the virus that originated in Wuhan, China, followed the announcement earlier this week of a man from Washington state who was diagnosed with the virus after returning from Wuhan.

The newly discovered virus has killed 26 people and infected more than 800. Most of the cases and all of the deaths so far have been in China, where officials have imposed severe restrictions on travel and public gatherings.

The virus has created alarm, but there are a still many unknowns surrounding it, such as just how dangerous it is and how easily it spreads between people. It can lead to pneumonia, which has been deadly in some cases.

“CDC believes the immediate risk to the American public is low at this time, but the situation continues to evolve rapidly,” Dr. Nancy Messonnier, director of the CDC’s Center for Immunization and Respiratory Diseases, said on the call.

More U.S. cases are likely, she added.

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Dr. Julie Gerberding, a Merck & Co executive who was CDC director during the 2003 SARS outbreak, said it is too early to predict the course the disease.

“I would caution against jumping to any premature conclusions about the severity of this because the epidemic is still unfolding,” she said in a phone interview.

“As we learn from basically every new outbreak, we don’t know where it’s going to end up until it gets there.”

The 60-year-old woman from Chicago had traveled to Wuhan in December and returned Jan. 13.

Dr. Allison Arwady, commissioner of the Chicago Department of Public Health, said on a conference call that the woman is in stable condition and remains in an unnamed Chicago hospital primarily for infection control purposes.

She had not taken public transportation and was not ill when she traveled, Arwady said.

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A few days after arriving home, the patient began to feel unwell and called ahead to alert her doctor to her illness. The physician asked about her travel history, quickly put a mask on the patient, and directed her to a hospital with infection control capabilities, Arwady said.

Hospital staff performed a full clinical examination and worked with public health officials to arrange testing for the novel coronavirus at the CDC.

Dr. Michael Ison, an infectious disease expert at Northwestern Medicine in Chicago, which is not caring for the patient, said the announcement of the second U.S. case means the system is working. “I don’t think the severity of the threat has changed,” he said.

In a local news briefing, Arwady said the health risk to Chicago residents remains low, adding there is no need for people to change their behavior in any way.

The World Health Organization on Thursday declared the virus an “emergency in China”, but stopped short of declaring it a global health emergency.