A man in Washington state has been diagnosed with a mysterious virus that began last month in China, becoming the first case confirmed in the United States of an illness that has killed at least six people and sickened hundreds more, according to U.S. officials.

The man, in his 30s, is in stable condition at Providence Regional Medical Center in Everett, Wash. Officials said they are monitoring him there out of an abundance of caution, not because he is seriously ill. The man arrived in the United States last week, before federal health officials began screening travelers from the central Chinese city of Wuhan at Los Angeles, San Francisco and New York's John F. Kennedy international airports, the first such effort since the 2014 Ebola outbreak.

Washington state health officials said the man, a resident of Snohomish County, Wash., returned from a trip to the region around Wuhan, where the outbreak began. He arrived at the Seattle-area international airport Wednesday. Shortly afterward, he began feeling ill and reached out to his health-care provider on Sunday. Local, state and federal officials quickly collected samples and sent them to the Centers for Disease Control and Prevention, where his case was confirmed Monday as the coronavirus, which has sickened nearly 300 people in China and others in Thailand, Japan, Taiwan and South Korea.

CDC officials said they are expanding screening to international airports in Atlanta and Chicago. More than 1,200 travelers have been screened since Friday. Over the weekend, federal officials began redirecting travelers arriving in the United States on direct and indirect flights from Wuhan to airports screening travelers for the new virus.

Even though the Washington state man's case was discovered several days before airport screening began, local, state and federal officials said they were on alert and quickly collaborated to test the man and put precautions in place. A small number of health-care workers and patients who may have been exposed to the patient have been notified to watch for symptoms and are being monitored, Washington state health officials said. Officials are tracing the people he may have come into contact with in China and the United States.

Officials praised the man for taking the initiative to reach out to his health-care provider. "He was a very astute gentleman," said Scott Lindquist, Washington state's epidemiologist. "He was looking at the Internet actively" and saw information about the virus and its spread, Lindquist said in a news briefing on Tuesday.

Nancy Messonnier, director of CDC's National Center for Immunization and Respiratory Disease, said the risk to the general American population is low, but older adults with underlying health conditions may be at increased risk. She said federal officials are prepared to respond quickly.

The outbreak has grown rapidly in recent days, with authorities in China confirming cases in multiple cities as hundreds of millions of people in China and elsewhere in Asia are on the move in the run-up to the Lunar New Year, the biggest migration event in the world. The infection is believed to have begun among people who shopped or worked at an animal market in Wuhan. But its rapid spread led officials to conclude that people and animals can transmit the infection.

Coronaviruses range from the common cold to much more serious diseases, which can infect humans and animals, according to the World Health Organization. The strain spreading in China is related to two other coronaviruses that have caused major outbreaks in recent years: Middle East respiratory syndrome, also known as MERS, and Severe Acute Respiratory Syndrome, or SARS.

"Since this is a respiratory virus, it is spread easier than Ebola so it brings with it more fear of easily being transmitted between people," said Matthew Frieman, a virologist and associate professor at the University of Maryland medical school who studies coronaviruses.

The WHO is meeting Wednesday to decide whether to declare the outbreak is a public health emergency.

Symptoms of a coronavirus infection include respiratory problems, difficulty breathing, fever and cough, and they can lead to severe cases of pneumonia, kidney failure, acute respiratory syndrome (when fluid builds up in the lungs) and death. The elderly, young children and those with an already weakened immune system are at a higher risk of developing severe lower-respiratory tract diseases, such as bronchitis and pneumonia, according to the Centers for Disease Control and Prevention.