Chinese health officials are warning that the death toll from the H7N9 bird flu is likely to rise in the weeks and months ahead.

In a report on the outbreak that began in China in February, doctors and researchers from several public health agencies said they suspected that most of the 82 people with confirmed cases of bird flu contracted the H7N9 virus from healthy-looking animals.

"To date, the mortality rate is 21 percent, but since many ... patients with confirmed H7N9 virus infection remain critically ill, we suspect that the mortality may increase," they wrote in their study, published online Wednesday by the New England Journal of Medicine. "Since this H7N9 virus appears to have emerged recently to infect humans, population immunity is expected to be low, and persons of any age may be susceptible to infection."

The report paints a fuller picture of the outbreak, which has caused Chinese people to become so panicked that one motorist felt the need to flag down police after a bird dropping landed on her car.

Cases of H7N9 infection have been confirmed in six provinces. The statistics paint a picture of the typical bird flu victim as an older urban male.

Chinese health officials tested 664 pneumonia patients who were sick enough to be hospitalized between March 25 and April 17. Of these patients, 81, or 12 percent, tested positive for H7N9 infection.

In addition, 5,551 unhospitalized people with flu-like symptoms were tested, and one of them was confirmed to have H7N9.

The 82 confirmed patients ranged in age from 2 to 89 years old, but most were at the higher end of that range and 46 percent were at least 65.

The report said that 73 percent of patients were men and 84 percent lived in urban areas.

Among the 17 people confirmed to have the virus who died, the median time from onset of illness to death was 11 days.

Their deaths were attributed to acute respiratory distress syndrome or multi-organ failure.

Sixty other patients confirmed to have the H7N9 virus remain critically ill, according to the report.

Until scientists are able to figure out exactly how the virus spreads from animals to humans, health officials should consider measures such as closing live poultry markets and culling the birds, the health officials wrote.