The exhalations of people with the flu can do more than just sicken others. Chinese researchers have developed a quick and noninvasive diagnostic tool that identifies influenza viruses in the breath of sick people. Doctors usually diagnose flu by its symptoms, but misdiagnoses are common. Reverse transcription quantitative polymerase chain reaction (RT-qPCR) can accurately detect the flu by identifying genetic material from viruses, but it takes several hours. To speed virus detection, Maosheng Yao of Peking University and colleagues developed a way to rapidly condense the liquid in breath, and then analyze it with an ultrasensitive silicon nanowire sensor.