A man in Washington state has been diagnosed with the 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, Wa. 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 screening since the 2014 Ebola outbreak.

The outbreak has grown rapidly in recent days, with authorities in China reporting confirmed 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, as well as animals, can transmit the infection.

Coronaviruses are a large family of viruses ranging from the common cold to much more serious diseases, which can infect both humans and animals, according to WHO. 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 School of Medicine who studies coronaviruses.

The World Health Organization is meeting Wednesday to decide whether to declare the outbreak a public health emergency.

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