At the same time, an even newer avian flu in China has killed its first human victim. That strain, known as H10N8, has been confirmed in only two people — a 73-year-old woman who died in December but whose infection was confirmed later, and a 55-year-old woman from the same province hospitalized in January. Both had contact with live poultry.

(Influenzas are described by the shapes of two protuberances on their surfaces: the hemagglutinin “spike,” or “H,” that attaches to cells, and the neuraminidase “helicopter,” or “N,” that chops off receptors, allowing new viruses to escape. There are 18 known types of H and 11 of N.)

While the newer influenza virus is worrisome, a mere two cases is “a far, far cry” from the risk posed by H7N9, said Ruben Donis, chief of preparedness for the flu division of the Centers for Disease Control and Prevention. He noted that the H7N9 virus had infected people in 10 Chinese provinces and was close to the borders of Vietnam, Cambodia and other poor Southeast Asian nations less adept than China at flu surveillance.

Birds appear to be spreading the H7N9 virus more through their breath than through feces, the normal infection route, Dr. Osterholm said, which means that fecal-sample testing in live markets may miss cases.

Monitoring the spread of the H7N9 and H10N8 viruses is difficult because neither makes chickens sick. Poultry farmers resist testing because a positive test forces them to destroy flocks that appear healthy. By contrast, the H5N1 virus rapidly kills flocks, so farmers want it extinguished.