China’s mysterious respiratory virus has caused six deaths and infected a number of medical workers, a sign the outbreak has entered a new phase with the illness spreading from person to person.


Health-care workers contracting the new illness indicates that it is more easily transmitted than previously thought, bringing the outbreak to a higher risk level, reminiscent of the severe acute respiratory syndrome, or SARS, pandemic that killed 800 people in Asia 17 years ago. China on Tuesday raised the number of confirmed cases to 291.As hundreds of millions of Chinese prepare to travel across the country and globally for the Lunar New Year holidays, concern is mounting that China will not be able to slow the spread of the pathogen, which originally emerged in the central city of Wuhan. It has since been found in people in Japan, Thailand and South Korea. Stocks in Asia slumped with China’s currency on the news, while haven assets rose.


“The risk from this virus causing pandemonium has increased because it is spreading from different countries, and we are now seeing that it can be more easily transmitted from person to person,” said Sanjaya Senanayake, associate professor of medicine at the Australian National University. In comparison with SARS, he said, “the one good factor, I guess, at the moment seems to be the low mortality rate.”


Both viruses belong to the coronavirus family. Fifteen medical professionals have been affected, with one critically ill, according to a report from China’s state news agency Xinhua. The transmission to medical workers is considered particularly worrisome because of the heavy precautions that were taken in Wuhan to try to minimize infections among health-care staff. Many doctors and nurses were also infected and died in the SARS outbreak.


Cities and provinces across China reported confirmed infections, including Shanghai, Tianjin, Zhejiang and Henan.


Wuhan, a city of 11 million, is now under heavy screening: people found to have symptoms like fever at travel checkpoints have been barred from boarding planes and trains. Tour groups have reportedly been banned from leaving the city. The city has reported 258 coronavirus cases as of Jan. 20, with 63 patients in serious or critical condition, mayor Zhou Xianwang told China’s state broadcaster CCTV.


Mounting fears about the deadly virus rocked financial markets, with Asia stocks slumping over the likely hit to retail and tourism during what should have been a peak period for spending. The FTSE China A50 Index of large caps Tuesday logged its biggest drop in six months.


Companies making diagnostic kits, air-cleaning equipment and plastic gloves saw shares surge across Asia.Despite the worries, the new virus is likely less deadly than SARS, said University of Sydney associate professor Adam Kamradt-Scott.


“It’s important to stress that this virus at the moment has been causing mild illness in the vast majority of people that have been affected,” he said in an interview on Bloomberg TV. “There’s around 10% of cases that have ended up in critical condition and there’s been deaths, but the vast majority of the 200-plus people infected have resulted in mild illness.”


Many viruses cross the barrier between animals to humans, as the Wuhan virus appears to have done, according to David Heymann, a professor of infectious disease epidemiology at the London School of Hygiene and Tropical Medicine who was formerly with the U.S.’s Centers for Disease Control and Prevention. Often it takes time to determine what form the new virus will take, he said.


Some viruses, like rabies, sporadically infect humans through animal contact but can’t spread from person to person. Others, like Ebola, emerge in small outbreaks, recede, and then reappear. Some of the most dangerous, like HIV, evolve into forms that infect humans in a widespread, persistent fashion.


The new virus “could be No. 2 or 3, that’s the concern,” Heymann said in an interview. “We need enough information to make a proper risk assessment.”


Heymann, who also serves on a World Health Organization committee that evaluates infectious hazards, said that it appears so far that the virus is most dangerous to elderly people who have existing disease. That would suggest it’s less of that a threat than SARS, which killed people of all ages, he said.


The WHO has set up three groups in Geneva to gather information and advise on the virus itself, the epidemiology of the outbreak and clinical findings from patients. A small team has also been sent to Wuhan to coordinate with local officials.


The exact source and transmission routes of the virus — known as 2019-nCoV — are still unknown. Some of the first group of patients in Wuhan worked or shopped at a seafood market where live animals and wildlife parts were reportedly sold.


These so-called wet markets, where shoppers mingle in narrow spaces with everything from live poultry to snakes, have been a key source in the emergence of new viruses transmitted from animals to humans.


Coronaviruses are a large family. Some cause illness in people, and others circulate among animals, including camels, cats, and bats, according to the CDC. In rare cases, animal coronaviruses can evolve to infect people and then spread between them.