A virus – previously unknown to science – is causing severe lung disease in China and has also been detected in other countries.
At least 17 people are known to have died from the virus, which appeared in the Chinese city of Wuhan in December.
There are already hundreds of confirmed cases, and experts expect the number will keep rising.
A new virus arriving on the scene, leaving patients with pneumonia, is always a worry and health officials around the world are on high alert.
But is this a brief here-today-gone-tomorrow outbreak or the first sign of something far more dangerous?
What is this virus?
Officials in China have confirmed the cases are caused by a coronavirus.
These are a broad family of viruses, but only six (the new one would make it seven) are known to infect people.
Severe acute respiratory syndrome (Sars), which is caused by a coronavirus, killed 774 of the 8,098 people infected in an outbreak that started in China in 2002.
“There is a strong memory of Sars, that’s where a lot of fear comes from, but we’re a lot more prepared to deal with those types of diseases,” says Dr Josie Golding, from the Wellcome Trust.