Coronaviruses are common, and typically cause mild respiratory conditions, such as a cough or runny nose.
But some are more serious - such as the deadly Sars (Severe Acute Respiratory Syndrome) and Middle East Respiratory Syndrome (Mers).
This outbreak - known as novel coronavirus (nCoV) - is a new strain that has not been previously identified in humans.
It seems to start with a fever, followed by a dry cough and then, after a week, leads to shortness of breath.
But in more severe cases, infection can cause pneumonia, severe acute respiratory syndrome, kidney failure and even death.
Most victims have been elderly people, suffering from other chronic diseases including Parkinson's and diabetes.
Peter Piot, professor of global health and director of the London School of Hygiene & Tropical Medicine, said the "good news" was that data suggested the virus may have a lower mortality than Sars.
There was also a diagnostic test and greater global sharing of information than previously, he said.
"And that is essential because you cannot deal with a potential pandemic in one country alone."
There is not yet a specific anti-viral treatment for the infection, so people with the virus are currently being treated for their symptoms.
You can do things to reduce your chances of catching it
The WHO is advising people in affected areas to folloe standard procedures to reduce the chance of catching the disease.
They include hand and respiratory hygiene as well as safe food practices.
People are advised to avoid close contact with people suffering from acute respiratory infections; wash hands regularly, especially after direct contact with ill people or their environment; and avoid unprotected contact with farm or wild animals.
Avoiding eating raw or undercooked animal products is also advised.
Those with symptoms of coronavirus should practise "cough etiquette", including maintaining distance, covering coughs and sneezes with disposable tissues or the inside of an elbow, washing hands and using hand sanitizer.
The WHO has said that while there is evidence of transmission between people in close contact, such as families or those in healthcare settings, there is not yet evidence of onward transmission.
If a case is suspected, there are processes to follow
The Chinese government has classified the outbreak in the same category as the Sars epidemic.
This means people diagnosed with the virus in the country must be isolated and can be placed in quarantine.
Within healthcare facilities, the WHO advises staff to implement enhanced standard infection provention and control practices, especially in emergency departments.
The WHO advises that patients should be assessed quickly and treated for the level of severity of the disease they have - mild, moderate, or severe.
It also recommends immediately implementing infection prevention measures. These include staff wearing protective clothing and limiting patient movement around the hospital.
Official guidance from Public Health England (PHE) says patients should remain in a room away from other patients and staff and be prevented from using communal toilets.
China coronavirus: A visual guide to the outbreak
By The Visual and Data Journalism Team BBC News