A new virus that has killed 17 people in China could be adapting and mutating, top Chinese officials said on Wednesday, while warning that it could spread further amid worldwide efforts to contain a possible wider outbreak.

China has confirmed over 500 cases of the previously unknown strain of coronavirus across more than 17 provinces, most of them in the country’s heavily populated central, southern and eastern regions.

The first suspected cases of the infection were reported from Hong Kong and Macau, both special administration regions (SAR) of China, and as far as Mexico on Wednesday. Over 100 people are under observation in Hong Kong.

Cases of the virus have also been detected in Thailand, Japan, South Korea, Taiwan and the United States, causing widespread alarm because of its similarity to SARS (Severe Acute Respiratory Syndrome), which killed nearly 650 people across mainland China and Hong Kong in 2002-2003.

Countries have been intensifying efforts to stop the spread of the pathogen — known by its technical name 2019 Novel Coronavirus (2019-nCoV) .

In India, a total of 43 flights and 9,156 passengers have been screened till Tuesday at seven airports, including those in Delhi, Mumbai, Chennai and Kolkata, Union health secretary Preeti Sudan said in New Delhi, adding that no cases were detected.

The government has directed these airports to make arrangements for screening passengers arriving from China.

Sudan said the Indian Embassy in China was providing the health ministry with updates on the status of the infection. She said passengers travelling from China were requested to report to the nearest public health facility if they develop symptoms such as fever, cough, shortness of breath, runny nose, and sore throat.

Britain said it will start enhanced monitoring of passengers who arrive on direct flights from the Chinese city of Wuhan, suspected to be the epicentre of the outbreak. Serbia, too, will screen passengers arriving at Belgrade’s Nikola Tesla airport using a thermal camera and is ready to quarantine anyone suspected of carrying the pathogen.

North Korea banned foreign tourists to guard against the entry of the virus, tour operators said. Most tourists to North Korea are Chinese or others who travel from China on organised tours.

Russia is working to develop a vaccine, Rospotrebnadzor, the country’s consumer safety regulator said on Wednesday, the RIA news agency reported.

The first case in the US — a Seattle resident in his 30s — is in good condition, American officials said. Mexican President Andres Manuel Lopez Obrador said authorities were investigating one potential case of coronavirus in the northern border state of Tamaulipas.

The virus is said to have originated from a seafood market in the central Chinese city of Wuhan in Hubei province, which, according to officials, was also dealing in “illegal wildlife”.

In Wuhan, large public events were cancelled on Wednesday and international football matches were moved to new locations. Visitors were urged to stay away, while residents advised not to leave the city of 11 million people.

An official said most of the 15 medical workers in the city who were infected caught it from a patient undergoing brain surgery who had contracted coronavirus. All 17 deaths in China have been reported in Hubei.

More provinces in China, including the southernmost Hainan province, reported the first confirmed cases of the disease on Wednesday.

The World Health Organization (WHO) was holding an emergency meeting on Wednesday to consider declaring the coronavirus a global health emergency.

Human-to-human transmission of the virus, which triggers a severe pneumonia, was confirmed by Chinese officials on Tuesday as experts continued to grapple with its source. Over 90% of the cases have been traced back to Wuhan and the seafood market.

The coronavirus is a large family of viruses that causes illnesses ranging from common cold to acute respiratory syndromes, but the virus that has killed people in China is a new strain.

The illness is mainly transmitted via the respiratory tract and there “is the possibility of viral mutation and further spread of the disease”, Li Bin, vice-minister of China’s national health commission (NHC), said at a high-level briefing in Beijing on Wednesday.

He added: “Recently, there has been a big change in the number of cases, which is related to our deepening understanding of the disease, improving diagnostic methods and optimising the distribution of diagnostic kits.”

Lin said 2,197 people who came into contact with infected people had been isolated, and 765 released from observation.

Gao Fu, head of the Chinese Centre for Disease and Control, said at the same briefing that the virus could already be “mutating and adapting”, and that made it “…particularly dangerous”.

There is evidence of community-based transmission and risk of “further spread of the disease,” Gao said.

With hundreds of millions of people travelling across China this week for the Lunar New Year holiday, the National Health Commission announced measures to contain the disease, including sterilisation and ventilation at airports and bus stations, as well as inside planes and trains.

Vice-minister Li said the festival rush would “increase the risk of the disease spreading and the difficulty of prevention and control”. At least three billion trips are expected to be made within China in a 40-day travel rush, which began on January 10.

As many 32 government departments and agencies are coordinating nationwide to control the spread of the disease, Li said.

Li added that there was as yet no evidence of “super-spreaders” capable of spreading the virus more widely, but authorities were already taking appropriate measures.

Fears of a pandemic initially spooked markets, with aviation and luxury goods stocks affected and the yuan falling, but they were regaining their footing on Wednesday in approval of China’s containment response.

Across China, companies from Foxconn to Huawei Technologies and HSBC Holdings were warning staff to avoid Wuhan and handing out masks. Terry Gou, the billionaire founder of Apple supplier, Foxconn, said he was advising employees not to visit China.

Should the virus develop dramatically and hit travel and growth, demand for oil could fall by 260,000 bpd, Goldman Sachs said in a note.

“Demand concerns over a potential epidemic will counter concerns around supply disruptions in Libya, Iran and Iraq, driving spot price volatility in coming weeks,” Goldman said, though the “impact on oil fundamentals remains limited so far”.