There are four more cases reported in a viral pneumonia outbreak in the central Chinese city of Wuhan.
Two people have already died. The total number of people who've contracted the illness is now 45 according to the Wuhan Municipal Health Commission. The cause has been traced to a new type of coronavirus.
It's prompted the US, Thailand, and Japan to take precautionary measures.
Airline passengers are being screened at three major airports arriving on flights from the city.
It comes as health authorities are keen to avoid a repeat of SARS which killed nearly 800 people in 2002. SARS is another coronavirus that spread to two dozen other countries.
Doctors started to see the new type of virus towards the end of last year in people who worked at or visited a food market in the suburbs of Wuhan late last month.
Symptoms include fever, cough, and difficulty breathing.
"On December 30th, China reported an outbreak of respiratory diseases in Wuhan, China," said Dr Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases.
"We're very concerned about this because it's attributed to a new coronavirus and that's a disease that we haven't seen before."
Extra people, around a hundred, have been deployed to take temperatures and ask about symptoms of incoming travellers at Los Angeles, San Francisco, and New York City's Kennedy airports.
It is a busy time to travel to and from China as it is the Lunar New Year which falls on Jan 25 this year.
It is thought pneumonia probably spread from animals to people but officials aren't ruling out the possibility of human-to-human transmission which would mean it would spread much faster.