Derided as dirty, dangerous and diseased, if there is one creature that tops the list of people's most hated animal, it is the rat. But rats are clever, and usually trainable. In Tanzania, they have been taught to detect patients with tuberculosis by detecting early signs in human saliva. What would take a human scientist a whole day to diagnose the disease takes a rat a mere seven minutes.

BBC Earth’s Extraordinary Animals series visited the social enterprise Apopo in Tanzania to see how the rats are trained, and discover what makes a rat's sense of smell so superior. A rat makes about eight sniffs a second compared with two for humans. Rats also smell in stereo, distinguishing two similar odours with one sniff. And around one in every 100 of a rat’s genes is involved with odour detection, compared with one in 1,000 humans.

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