A new strain of bird flu is circulating in China, and authorities are keeping a close eye on a potentially disastrous scenario. On Aetiology, Tara C. Smith writes that by now, “the microbe may have already become established in the population, adapting to humans stealthily before we were even aware of it.” Greg Laden writes, unlike H1N1 in 2009, the new H7N9 doesn’t sicken birds, making it more difficult to identify reservoirs of the virus. And according to the latest reports, it doesn’t make all people sick either. Documented infections are widespread in a populous region, and of more than 100 known cases, twenty people have died. So far there is no reason to panic—but the real threat lies in further mutation of the virus, and the emergence of a killer global flu.