Cases of the Wuhan coronavirus have increased dramatically over the past week, prompting concerns about how contagious the virus is and how it spreads.

According to the World Health Organisation, 16-21% of people with the virus in China became severely ill and 2-3% of those infected have died.

A key factor that influences transmission is whether the virus can spread in the absence of symptoms – either during the incubation period (the days before people become visibly ill) or in people who never get sick.

On Sunday, Chinese officials said transmission had occurred during the incubation period.

So what does the evidence tell us so far?