Emerging out of virtually nowhere, the new strain of swine flu which has combined with human strains, apparently enabling human-to-human transmission, poses a threat of a pandemic-level infection, warns the World Health Organization (WHO). The rash of infections in Mexico and the United States not only tests the public health response in those countries but poses a wider risk of spreading, particularly as people seem to have been incubating the disease for some days before showing symptoms. The WHO's director general Margaret Chan said the outbreak in Mexico and the United States constituted a "public health emergency of international concern". Britain's The Times reported that the disease - a variant of the H1N1 virus responsible for the Spanish Influenza of 1918 - had likely been spreading in the Mexican population for weeks: A deadly strain of flu that combines elements of swine, avian and human viruses could spread around the world after emerging simultaneously in Mexico and the United States, experts warned yesterday. A British Airways cabin crew member was taken to hospital with flu-like symptoms yesterday afternoon after falling ill on a flight from Mexico City to Heathrow. The Health Protection Agency said it was keeping a close eye on the situation. Britons are not being advised to avoid travelling to affected areas, although anyone visiting those destinations or who has recently returned should consult a doctor if they experience flu-like symptoms. Mexican soldiers distributed surgical masks and the authorities warned people to avoid shaking hands and kissing. The mayor of Mexico City cancelled all public events, and schools, museums, cinemas and libraries were closed. Health workers were posted at airports and at stations to keep anyone showing symptoms off public transport. US scientists said Mexicans had been dying for weeks before the virus - an animal strain of H1N1, the virus that killed 50m in the Spanish flu pandemic of 1918 - was detected. Controlling people's movements might not now keep it from spreading, they said. "It is clear that this is widespread," said Dr Anne Schuchat of the US Centers for Disease Control and Prevention. The New York Times reported that the high school infections in Queens were likely linked to recent visits by the students to Mexico, highlighting the risk of spreading the disease entailed in air travel: Tests show that eight students at a Queens high school are likely to have contracted the human swine flu virus that has struck Mexico and a small number of other people in the United States, health officials in New York City said yesterday. The students were among about 100 at St Francis Preparatory School in Fresh Meadows who became sick in the last few days, said Dr Thomas R Frieden, New York City's health commissioner. "All the cases were mild, no child was hospitalised, no child was seriously ill," Dr Frieden said. Health officials reached their preliminary conclusion after conducting viral tests on nose or throat swabs from the eight students, which allowed them to eliminate other strains of flu. Officials were also suspicious since some St Francis students recently had been to Mexico, where the outbreak is believed to have started. The president in Mexico assumed emergency powers to deal with the crisis, which has killed at least 81 people and infected about 1,300 others. All public gatherings have been banned, including more than 500 concerts and sporting events and the popular bicycle rides on closed boulevards. The WHO convened an emergency meeting of experts on Saturday, but the panel adjourned without raising the global pandemic alert level, saying it wanted more information. Some experts expressed surprise that no action was taken since the Mexico outbreak seems to meet the definition of a Level 4 alert ? sustained human-to-human transmission of a new virus. The alert has been at Level 3 for years because of small clusters of human cases of avian flu. In the United States, so far, at least 11 swine flu cases have been confirmed. Seven were confirmed in San Diego and Imperial Counties in California and two in Kansas. In Texas, two 16-year-old students at Byron Steele High School in Cibolo, near San Antonio, were confirmed to have swine flu, and one of their classmates was suspected to have the virus. There have been no deaths, and officials said most of the 11 seemed to be recovering. Officials said they expect to find many more cases as they begin testing for them. In New York, the samples from the Queens students have been sent to the Centers for Disease Control and Prevention in Atlanta, the only lab in the country that can positively confirm the new swine flu strain ? which has been identified as H1N1. Results were expected on Sunday, officials said. Fearing a panic that might tax local health facilities, Dr Frieden urged New Yorkers not to go to a hospital if they had typical mild cold or flu symptoms. If they are seriously ill, especially with lung problems, they should seek medial attention promptly, he said, because antiflu drugs work best if taken in the first 48 hours. Because of fears of the H5N1 avian flu, both New York City and the United States have had detailed pandemic emergency plans in place since 2005, as well as stockpiles of emergency supplies and flu drugs (the plan can be read at www.pandemicflu.gov). Dr Frieden said that for such an emergency, the city had extra hospital ventilators, huge reserves of masks and gloves and "millions of doses of Tamiflu", an antiflu drug that thus far appears to work against the new swine strain. The eight Queens students were positive for an A-strain flu virus but negative for all previously known A-strains. That result, called "A-untypeable", led officials to suspect it was the new swine flu. Bloomberg reported that US President Barack Obama had come in contact with a man who later died of flu-like symptoms during his recent trip to Mexico: Mexican President Felipe Calderon declared an emergency in his country's swine flu outbreak, giving him powers to order quarantines and suspend public events. Authorities have cancelled school at all levels in Mexico City and the state of Mexico until further notice, and the government has shut most public and government activities in the area. The emergency decree gives the president authority to take more action. "The federal government under my charge will not hesitate a moment to take all, all the measures necessary to respond with efficiency and opportunity to this respiratory epidemic," Calderon said today during a speech to inaugurate a hospital in the southern state of Oaxaca. The first case was seen in Mexico on April 13. The outbreak coincided with the President Barack Obama's trip to Mexico City on April 16. Obama was received at Mexico's anthropology museum in Mexico City by Felipe Solis, a distinguished archaeologist who died the following day from symptoms similar to flu, Reforma newspaper reported. The newspaper didn't confirm if Solis had swine flu or not.