With the cause of infection and transmission still unknown, the number of deaths due to the new influenza A H7N9 virus has risen to 20. The number of infected likewise jumped to 102 over the weekend.

The Chinese Centre for Disease Control and Prevention said 40 per cent of the infected patients had no direct contact with poultry, making experts wonder just how the sick got infected with the new virus.

However, fears rose that the virus could pass between humans after a number of members of a family in Shanghai got infected.

"To me, the biggest question is the link between the virus in birds and how it gets to humans. This is not clear," Dr Bai Chunxue, a Shanghai-based respiratory expert, was quoted by the South China Morning Post. Dr Bai was the one who treated a family cluster, an 87-year-old man and his two sons, when they got sick due to the influenza A H7N9 virus.

Dr Bai said the patients had no contact with birds or poultry. "So this is indeed a mystery," he said.

And just last week, China reported the first ever human carrier of the new influenza A H7N9 virus, a 4-year-old boy in Beijing. His parents are poultry and fish traders. Even without showing symptoms of the potentially lethal virus, China reported the child carries the disease.

Of the 102 total infected cases, 70 remain in hospital while 12 have been discharged. Five of the new cases were in Zhejiang province and one came from Shanghai.

"Until the source of infection has been identified, it is expected that there will be further cases of human infection with the virus in China," the World Health Organisation (WHO) said in a statement.

With 40 per cent of patients having no exposure to poultry or other birds, this virus is "very difficult to understand," Dr Masato Tashiro, director of WHO's influenza research center in Tokyo, said.

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