A second patient in the United States has been found to be infected with the Wuhan coronavirus, the Centers for Disease Control and Prevention announced on Friday.
The patient is a woman in her 60s who had traveled to Wuhan, China — the epicenter of the outbreak — and returned to Chicago on Jan. 13. She did not become ill until several days after arriving in Chicago, so officials do not think there is much risk to others who were on her flight home.
She did not take public transportation, attend any large gatherings or have close contact with anyone outside her home after returning, officials said. Members of her household are being monitored.
The woman was taken to a hospital, but she is doing well and is being kept there primarily to keep the infection from spreading, officials said at a news briefing. They declined to name the woman or the hospital.
The first known case in the United States, of a man in Washington State who had traveled to Wuhan, was reported on Tuesday.
“The immediate risk to the American public is low at this time, but the situation continues to evolve rapidly,” Dr. Nancy Messonnier, the director of the National Center for Immunization and Respiratory Diseases, said at a news briefing held by the C.D.C. More cases are expected in the United States, she said.
Sixty-three other people in 22 states are being monitored for possible infection, the C.D.C. said. They are people with respiratory symptoms who may have been exposed to the virus through their own travels or by contact with a patient. So far, 11 of the 63 have tested negative for the infection.
The people being tested, called “patients under investigation,” were reported to the C.D.C. by state health officials. Samples from the patients — respiratory fluids and blood — have to be sent to the C.D.C., which tests them for genetic material from the virus.
The testing takes four to six hours. The C.D.C. is developing kits that officials will send out to the states so that local authorities can do the testing themselves and get results faster.
Following a briefing in Washington by officials from the C.D.C. and the National Institutes of Health on Friday, senators said it was most likely that more cases would be identified in the United States.
“The idea is to identify and then contain it,” said Senator John Barrasso, Republican of Wyoming.
“We’re trying to stop and prevent anyone from coming to the United States that may have it,” he added.
The C.D.C. is sending a team to talk to the patient and her family in Chicago, according to Senators Richard J. Durbin and Tammy Duckworth, both Democrats of Illinois.
Coronaviruses are worrying because epidemics caused by other members of the viral family, SARS and MERS, have had high death rates: 10 percent for SARS, and about 35 percent for MERS. It is too soon to know the death rate for the new virus.
Although influenza kills more people every year, its overall mortality rate is only about 0.1 percent. The number of the deaths is high because so many people become infected. The death rate of the Spanish flu in 1918 was about 2 percent.
Because of the higher death rates, widespread coronavirus epidemics could take a heavy toll, so the health authorities scramble to stamp them out.
A major concern is that with both SARS and MERS, a few patients inexplicably became “superspreaders” who infected huge numbers of other people. At a hospital in Seoul, South Korea, in 2015, one man with MERS transmitted it to 82 patients.
The C.D.C. now recommends avoiding nonessential travel to Wuhan.
Because the disease has spread beyond Wuhan, travelers to other parts of China should avoid contact with people who are sick, stay away from markets that sell live wild animals — believed to be the source of the virus — and wash their hands frequently.
Federal health officials are asking people with possible exposure to the virus and who develop symptoms like shortness of breath, cough and fever to call health care providers first, rather than just showing up at a doctor’s office, clinic or emergency room.
With advance notice, doctors can take steps to prevent the spread of the virus, like isolating patients and putting masks on themselves and the patients. The woman in Chicago followed those instructions, officials said.
There is no antiviral drug to treat coronavirus infection, but a medicine called remdesivir has shown promise in animal studies and would be a good candidate for trials in humans, according to Dr. Mark R. Denison, an infectious disease expert at Vanderbilt University.
There is also no vaccine to prevent the disease, but researchers at the National Institutes of Health reported on Thursday that it might be possible to develop one ready for testing in people within a few months.
The incubation period of the disease — the time it takes for symptoms to develop after a person has been exposed to the virus — is thought to be about two weeks, Dr. Messonnier said.
Nearly 1,300 people are infected in China, and a handful of cases have turned up in at least seven other countries. All were travelers who brought home the infection from China. At least 41 people have died.
On Thursday, the World Health Organization declined to declare the outbreak a global emergency, but it “may yet become one,” said Dr. Tedros Adhanom Ghebreyesus, the W.H.O.’s director general. It is certainly an emergency within China, he added.
Five major airports in the United States are screening passengers arriving from Wuhan for signs of infection with coronavirus.
The screenings are imperfect. The illness may resemble a common cold or the flu. Some infected travelers, like the two who have been found to have the disease, may have arrived in the United States before symptoms were evident or before screenings were implemented.
People being screened are also given cards with instructions about what symptoms to look out for and how to seek health care if they develop them.
Emily Cochrane contributed reporting from Washington.