But each case represents a chance for the virus to mutate into one that is highly infectious in humans. And it is an unusual strain — normally avian diseases make birds sick first, giving an early warning sign, but this one does not.

Scientists have established it is from an “avian reservoir” but still don’t know the precise source. Chinese officials have dismissed suggestions of a connection with the large number of dead pigs and other animals found recently in rivers.

Many in China are understandably worried, with some deciding to avoid eating chicken, even though it poses no threat if properly cooked.

KFC’s parent company Yum reported on Wednesday that sales in its Chinese restaurants had dropped by 13 percent in March, saying “publicity associated with avian flu in China has had a significant, negative impact.”…

Work has already started in the U.S. to make a vaccine against the new strain — just in case.