A top mainland scientist said 40 per cent of the people who have tested positive for the new, deadly strain of bird flu had no recent contact with poultry, and so it is still unclear how they contracted the virus.
The remark by Dr Zeng Guang , chief of epidemiology at the Chinese Centre for Disease Control and Prevention, came as Shanghai and Zhejiang authorities announced five new cases of H7N9, bringing the number of confirmed cases to 82, with 17 of them fatal.
"How were they infected? It is still a mystery," he was quoted by The Beijing News yesterday as saying. All the cases so far have been in China.
The Ministry of Health cited Feng Zijian, director of the CDC's office of emergency response, as saying the source of the illness was hard to pinpoint. He noted that during the last major bird flu outbreak, which involved H5N1, half of the victims could not remember whether they had come in contact with poultry or other birds. Still, he said he believed that all infected during this outbreak must have come in contact with an environment contaminated by fowl, or in contact with birds directly.
Of the five new cases, just one was in Shanghai - an 89-year-old man from Jiangsu province. Authorities did not give his condition. The first person infected in Beijing, a seven-year-old girl, left hospital yesterday.
A four-year-old boy in the capital infected with the strain but not displaying symptoms remained in quarantine.
Health authorities hope the boy will help doctors understand the virus. He was identified as a carrier via a random blood test of people in or near the poultry industry.
Hunan this week became the sixth major region to officially announce a case of H7N9 - a two-year-old child whose sex was not given. The child and his parents travelled from Shanghai to Changsha, Hunan, on March 17, when he began running a fever, according to Hunan health authorities. He was admitted to hospital in Changsha and returned to Shanghai two days later. He was again admitted to hospital, where he recovered. He was discharged on March 22.
Meanwhile, a research team in Hubei province has a new theory regarding the evolutionary path of the bird flu strain.
Professor Xue Yu, a biologist at Huazhong University of Science and Technology in Wuhan , said researchers believed the deadly virus evolved after Chinese birds came in contact with birds from East Asian countries such as South Korea.
Xue's analysis was in line with other studies on the mainland, including by researchers at the Chinese Academy of Sciences, that suggested the decisive element of the deadly virus came from East Asian countries.
But it differs from a theory put forward by Japanese researchers, which blames birds migrating from Europe.
View H7N9 map  in a larger map
Click on each balloon for more information on individual patients infected with the avian flu virus: blue, patients infected with the H7N9 virus under treatment; red, those infected with H7N9 who have died; and pink, those infect with the H1N1 avian flu virus.