H7N9 Bird Flu Cases in China Rise to 91, Transfer Method Still Unkown

The H7N9 bird flu strain has already infected 91 people in China and killed 17, according to local media reports, and researchers are still investigating the potential source of the virus.

The first human case was identified three weeks ago, and the rapid compilation of human cases since then has public health officials in China and scientists from around the world scrambling to identify the source of the infection and prevent further spread.

Scientists believe the H7N9 bird flu strain does not appear to transfer easily between humans. Human-to-human transmission is a critical sign that a virus could reach pandemic levels, such as what occurred with the H1N1 "swine flu" that quickly spread around the world in 2009.

More than half of the victims have had no contact with poultry, the state-run Shanghai Daily reported Friday. "This is still an animal virus that occasionally infects humans," the newspaper quoted World Health Organization's China leader Michael O'Leary as saying.

Over the past decade, H5N1 has caused sporadic, limited infections among humans in Asia and the Middle East, affecting almost 700 people, killing 60 percent of them.

Most troubling, H7N9 could become fully adapted to humans and touch off a global pandemic.

Shanghai, the city with the most bird flu deaths so far, is playing host next week to one of the country's most important auto industry conferences this year.  

Chinese officials have been aggressively screening people who came into contact with patients who became sick. That screening has identified at least two families with multiple people who were sick, although it's not clear whether the disease spread from one family member to another or if the people became sick from the same source, such as an infected bird.