A man returning from China has become the first person in the US to be diagnosed with a mystery new virus.
The American citizen, in his 30s, is in good condition in hospital near Seattle after returning last week, according to health officials.
He did not show any symptoms when he arrived back on Wednesday, but contacted doctors on Sunday when he felt unwell.
He is not considered a threat to medical staff or the public.
Nine people have died from the illness in Wuhan, the capital of China's central Hubei province, most of them aged 60 or older, including some with a previous medical condition.
"We do expect additional cases in the United States and globally," said Dr Nancy Messonnier, a respiratory diseases expert at the Centers for Disease Control and Prevention (CDC).
It has begun tracking down people who came in contact with the man to check them for symptoms.
Four hundred and forty people have been infected so far, with cases identified in Thailand, Japan, South Korea and Taiwan - all of them had been in China.
This new coronavirus virus - which can cause coughing, fever, breathing difficulty and pneumonia - originated at a seafood market in Wuhan.
Coronaviruses are a large family of viruses ranging from the common cold to severe diseases such as SARS - which killed nearly 800 people during the 2002-03 outbreak.
When a new strain emerges that has not yet been identified, as with the current outbreak, it becomes known as a novel coronavirus (nCoV).
Chinese state media say the latest strain in different from those identified in the past.
The outbreak has already spread to other Chinese cities including Beijing and Shanghai.
Officials in China confirmed on Monday that the virus, which has no cure, can spread between humans .
Concerns are growing as hundreds of millions of people in the country prepare to travel for the week-long Lunar New Year holiday which starts on Saturday.
The (WHO) has called an emergency meeting for Wednesday to consider declaring an international health emergency, a move only used for the worst epidemics.
Fears about a global outbreak similar to SARS have prompted some countries to introduce screening for travellers from China, especially those arriving from Wuhan.
Health screenings have started at three international airports in the US - New York's JFK, San Francisco and Los Angeles.
They are initially expected to involve around 5,000 passengers, according to the CDC.
Chicago's O'Hare airport and Atlanta's airport will also begin screenings later this week.
Officials are expected to begin to force all passengers originating from Wuhan to go to one of these five airports if they want to enter the US.
Several other nations, including Italy, India and Australia are also introducing checks, while North Korea is planning to temporarily ban foreign tourists, who are mainly Chinese, according to a foreign tour operator.
However, University of Washington coronavirus researcher David Veesler said people "should not be panicking right now".
He claims the response has been "very efficient" and that "in a couple of weeks, China was able to identify the virus, isolate it, sequence it and share that information".
"We don't have enough data to judge how severe the disease is," he added.