How quickly coronavirus spreads beyond China?

For WHO chief Tedros Adhanom Ghebreyesus, the key question in the battle against the novel coronavirus is how quickly it is spreading beyond China. "The detection of a small number of cases may indicate more widespread transmission in other countries; in short, we may only be seeing the tips of the iceberg," he warned in a recent tweet.

Here are some of the key issues: How the virus transmits

It is a respiratory virus, spreading through contact with an infected person: their coughs, sneezes or droplets of saliva or discharge from the nose.

Scientists think a person needs to be within a metre of an infected person to risk catching it.

Health officials stress basic precautionary measures: washing hands frequently, covering up coughing or sneezing. And if you are infected, wear a mask to protect others.

Diarrhoea may be a secondary mode of transmission, according to one team of researchers.

And Chinese scientists writing in the New England Journal of Medicine suggested every person infected would, on average, infect another 2.2 individuals.

That is a higher rate than winter flu (1.3), lower than an infectious disease such as measles (more than 12) and comparable to SARS (3) - the last major virus that broke out in China, in 2002-03.

When it becomes contagious

Scientists initially thought the virus became contagious several days after symptoms started to appear, as happened with SARS, Arnaud Fontanet of the Institut Pasteur told AFP.

They now think it could be infectious earlier than that.

"Today, everybody agrees that the contagious period starts as soon as symptoms appear," said Fontanet, a specialist in tracking emerging diseases.

There had even been a few cases of transmission from people who showed no symptoms, he added.

One reason these cases are rare is that coughing is a major means of transmission - and a symptomless carrier does not cough. Even so, transmission by symptomless carriers makes it harder to contain the virus because they are harder to spot.

Spread outside China

The concern expressed by Ghebreyesus about the spread of the virus beyond China came as news broke of a British national infected in Singapore.

He went on to infect several other Britons during a stay at a French Alpine ski resort, before finally being diagnosed back home in England.

By then, he had infected at least 11 people, including a nine-year-old child.

"What is concerning is the source of these infections, which appears to have been a conference in Singapore attended by some participants from China," said Nathalie MacDermott of Imperial College London.

Some of the 90 other people there might also have taken the virus back to their home countries, said MacDermott, a specialist in paediatric infectious diseases. Every time health officials identify an infected person, they have to track down those they might have infected to ensure they, in turn, are not passing it on, she said.

She and other experts are already worried about "the potential for this epidemic to develop into a pandemic".