The World Health Organization (WHO) has confirmed that it has managed to enlist China in the international investigation of the mystery pneumonia known as severe acute respiratory syndrome (SARS).

But even as agreement was reached, leading Chinese researchers clashed with WHO investigators over the probable cause of the illness.

SARS arose in East Asia and, according to WHO figures on 31 March, it has so far spread to 13 countries, infecting 1,622 people, 58 of whom have died. One of the recent casualties was Carlo Urbani, an Italian physician working for the WHO, who identified the first case of the disease while treating patients in Vietnam.

Researchers and public-health officials now assume that SARS is the same disease as that which struck the Guangdong province in southern China late last year. Initially, Chinese officials said that the epidemic had infected about 300 people and had petered out in February (see Nature 422, 247; 200310.1038/422247a). But last week, they admitted that by the end of February it had infected at least 806 people, causing 34 deaths.

An international investigation coordinated by the WHO has identified two virus families — coronaviruses and paramyxoviruses — as the possible cause of the epidemic, although some believe that the illness may involve a combination of both families. But Tao Hung, a researcher at the Institute of Virology at the Chinese Academy of Preventative Medicine in Beijing, who is heading the Chinese investigation of the epidemic, says that his results point in another direction — a Chlamydia-like bacterium. He says he has found traces of Chlamydia in samples from organs taken from five people who died from SARS and from 30 infected patients.

But researchers inside and outside China dispute that the normally non-lethal Chlamydia could exact such a heavy toll. “It would go against common sense,” says Masato Tashiro of the National Institute of Infectious Diseases in Tokyo, one of the WHO's investigators.

“From a clinical point of view, it cannot be Chlamydia,” says Nan Shan Zhong, a doctor at Guangzhou Medical College in Guangdong, who is advising the government on the outbreak in the province. He adds that antibiotics against Chlamydia have proven ineffective in treating the syndrome. “I do not believe the disease is really under control,” he says.

In the agreement reached on 30 March, a WHO delegation led by John Mackenzie, a virologist at the University of Queensland, Australia, convinced China to become a full participant in the WHO operation. This will mean full updates, sharing resources, and allowing WHO officials to visit doctors in Guangdong — something that has been repeatedly promised but has yet to occur.