HONG KONG — Of all the mysteries surrounding the emergence of a new and deadly strain of avian influenza around Shanghai, one of the biggest is why China’s hundreds of medical and veterinary labs did not spot the problem sooner — or if they did, why it was not disclosed.

Even the censored Chinese news media has begun cautiously questioning why the authorities did not say anything sooner about a disease that resulted in the first known human case in eastern China on Feb. 19, but was not announced to the public until March 31. The announcement came two weeks after the closing of the National People’s Congress, a show event during which the Communist Party traditionally avoids acknowledging problems.

“People are still asking, why did it take the government so long to confirm the outbreak?” The Communist Youth Daily, the official newspaper of the Communist Youth League, said in a column several days ago. “The transparency of information from the government is still being called into question by the public, and the actions the government has taken have not convinced the public.”

China’s health ministry is now finding three to five human cases a day, a brisk pace for a disease that Chinese officials and the World Health Organization assert is still transmitted from animals to people, and not from person to person. The Centers for Disease Control and Prevention in Atlanta activated its emergency operations center this week as governments around the world began making preparations in case of a flu pandemic.