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Humans have long taken advantage of dogs’ superior sense of smell. The 300 million scent receptors in a dog’s nose are routinely used to detect bombs, drugs, firearms and people. But dogs are now being used experimentally to sniff out human disease. Cynthia Otto, a veterinarian and director of the Penn Vet Working Dog Center at the University of Pennsylvania in Philadelphia, spoke to Nature about how sniffer dogs could help in improving medical diagnostics.