As China tries to stop people leaving the city of Wuhan in an attempt to control a deadly virus outbreak, millions of Chinese have already embarked on the world’s biggest annual migration to travel home for the lunar new year holiday.

Authorities have revealed a jump in the number of people infected. The newly identified coronavirus has already killed 17 people and 571 people have tested positive for the pathogen, according to state television on Thursday.

Chinese health officials on Wednesday warned that the Sars-like virus could be mutating and that there could be further spread of the disease.

A team at the University of Hong Kong estimated on Tuesday that almost 1,700 people in Wuhan may have been infected with the virus, and that it would have spread to 20 Chinese cities based on travel patterns and the cases that had so far been found overseas. Prof Gabriel Leung, chair of public health medicine at the University of Hong Kong, said he expected a “substantial upward revision” to the estimate following news of confirmed cases in more overseas countries.

Other scientists who have been modelling the virus’s spread told a press briefing in London on Wednesday that 4,000 people could have been infected in Wuhan. Neil Ferguson, professor of mathematical biology at Imperial College in London, added that the margins of error meant that up to 9,700 people could be carrying the virus. He cautioned that this alarming figure might be due to a “massive ramping up of surveillance and testing” in China rather than the virus itself being particularly infectious. The briefing also heard reports of “superspreading” events, in which individuals transmit the disease to many more contacts than is usual. Superspreading was also seen in the Sars outbreak, with one patient infecting about 100 others.

Where has it spread internationally?

US health officials on Tuesday confirmed that the first case of a person infected with the new coronavirus, 2019-nCoV, was a man in his thirties just north of Seattle. He had travelled from Wuhan to the US.

Hong Kong and the neighbouring Chinese territory of Macau reported their first cases on Wednesday — both travellers from Wuhan. Japan, South Korea and Taiwan have identified cases over the past week, all of which have been linked to the city in central China. Thailand has confirmed four cases of the virus.

Prof Ferguson added that the outbreak met the criteria for declaring a Public Health Emergency of International Concern. But airport screening for fever would not guarantee containment, the briefing heard, given that the incubation period, though currently unknown, could be about five to six days, in line with Sars and Mers. The growth in air traffic between China and Africa is one focus of concern; scientists fear that the arrival of the virus on the continent could overwhelm health systems.

How are people catching it?

Chinese health authorities say evidence suggests the disease is transmitted through the respiratory tract.

Early cases of the virus were linked to a market in Wuhan that sells animals as well as seafood. The market was closed at the start of January in an attempt to contain the outbreak. Live chickens and wild animals destined for sale have now been banned from the city.

After the initial outbreak, other patients were found that had not been in contact with the market.

“Our experts believe the cases are mostly linked to Wuhan,” said Li Bin, vice-minister of China’s National Health Commission, on Wednesday. “There has already been human-to-human transmission and infection of medical workers and there is community-based transmission at a certain scale.”

Prof Leung said the biggest questions were whether sustained human-to-human transmission would take place between people with no connection to Wuhan and if there was now more than one epicentre.

“Given those upward adjusted numbers, are they going to turn into a second or third epicentre? And . . . the containment strategy within Wuhan, is that possible? That is, I think, the big unknown,” he said.

How have Chinese authorities responded?

Chinese authorities shut down transport networks and suspended outgoing flights from the city of Wuhan from Thursday morning. The entire province of Hubei, of which Wuhan is the capital, has been told to intensify its emergency response, and wild animals and live poultry destined for sale have been banned from the city.

Temperature checking equipment has been deployed across China to test people at airports, train stations, coach stations and ports.

A strict isolation policy has been introduced for patients with a temperature, as well as for their close contacts.

The whole country must now make daily reports, even if there are no new cases, Mr Li said on Wednesday.

Health authorities said anyone with a fever, coughing or difficulty breathing who had either visited Wuhan, or was in touch with someone who had visited the city, should seek hospital treatment. The public was reminded to wash their hands regularly and to wear a mask in hospitals or clinics.

How did the outbreak start?

Gao Fu, head of China’s Center for Disease Control and Prevention, said on Monday that evidence had been found linking the new coronavirus to wildlife at the market, according to China Daily. However, the animal involved had not been identified.

A 2002-2003 outbreak of severe acute respiratory syndrome (Sars), the deadly virus that killed more than 800 people, was passed to humans via contact with civet cats.

Is it the same as Sars?

Experts have cautioned that there are important differences between the two coronaviruses. While everyone who contracted Sars suffered severe illness, current symptoms range from mild to deadly, with the 571 known cases in China possibly representing the most extreme end of the spectrum. This raises the worrying possibility that some individuals are unknowingly carrying the virus, putting their contacts at risk. The wider the spread, the greater the risk the virus will mutate.

Those who have sought treatment show a consistent pattern of symptoms: an initial fever, dry cough, and, within a week, difficulty with breathing. Up to one in five who seek medical help will end up requiring ventilation or life support. There are no antivirals or other treatments; patients either recover or die. All those who have been hospitalised are over 40. Prof Peter Horby, from the University of Oxford, said the two key uncertainties were how transmissible the virus was between people, and how deadly: “These will determine the risk the world faces at the current time.”