BEIJING/LONDON : Health officials raised further questions on Friday about the source of a new strain of bird flu infecting humans in China after data indicated that more than half of patients had had no contact with poultry.The H7N9 virus has been found in 87 people, mostly in eastern China, and killed 17. But it is not clear how people are becoming infected and the World Health Organisation (WHO) says there is no evidence of the most worrying scenario - sustained transmission between people.The WHO's China representative, Michael O'Leary, issued data on Friday showing that half of the cases analysed had had no known contact with poultry, the most obvious potential source, but he said it appeared human-to-human transmission was rare. "This is still an animal virus that occasionally infects humans," he said.