"There are still many unknown aspects of the H7N9 bird flu virus as well as diseases that might result from it, including the source of the virus, the infection routes and mutation."

Wang Yu, director of the Chinese Center for Disease Control and Prevention, speaking at an international symposium on the prevention and treatment of new types of influenza. Wang said H7N9 bird flu has killed 45 people on the Chinese mainland since the first human infection was confirmed in late March. A total of 134 H7N9 infections have been confirmed on the mainland.