“This is how it started,” Church told the inaugural international conference on medical biodetection, held in Cambridge, U.K., in September 2015. “It was all anecdotal.”

At least, that was how interest in using dogs to sniff out cancer began. But the idea of smelling breath, urine and stools to diagnose disease goes back millennia. In the time of Hippocrates, around 400 BCE, it was reportedly common for patients to cough and spit on hot coals to generate a smell that the physician would sniff to aid diagnosis.

Methods for disease diagnosis have clearly come a long way. But the Lancet letters got some, including John Church, thinking: Might animal noses be quicker, or more accurate, or cheaper—and so able to be used more widely—than some high-tech cancer-screening techniques? If dogs really could sniff out cancer, what other diseases might they smell? And might the noses of other animals be useful too?

Over the past decade, there have been projects investigating the use of bees to sniff out cancer, for example, but that research hasn’t advanced very far. The overwhelming focus in the field now is on dogs—and the African rats.

When someone with TB coughs, he or she exhales compounds produced by the bacterial pathogen Mycobacterium tuberculosis. If the TB is advanced enough, the smell of these compounds can even be detected by people. In 2002, when research to investigate the potential of using dogs in cancer diagnosis was in its embryonic stage, a former product designer from Belgium called Bart Weetjens began wondering about African giant pouched rats and TB.

Weetjens already knew that TB has a distinctive smell. “There is a lyric of a Van Morrison song: ‘I can smell your TB sheets’—your bedsheets.” Also, “in my native language, Dutch, the name for TB traditionally is tering, which etymologically refers to the smell of tar.” Weetjens also knew that these rats are superlative sniffers. More than that, he understood how to breed them and how to train them, and his track record of using this species to save lives, albeit in a very different setting, was well-established.

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As a boy, growing up in Antwerp, Belgium, Weetjens had kept pet rats. “Not only rats—I was very fond of all kinds of rodents. Hamsters and mice, and then rats. I tried gerbils and squirrels as well.” He bred them in his bedroom. “I learned that they smell very well, but I was not occupied with that. I was simply breeding these animals to give offspring to the pet shops. It was a way to get pocket money. I gave up all rat breeding in my bedroom when I was 14.”

After graduating and starting work as a product designer, Weetjens found himself increasingly preoccupied with the problem of landmines. “I saw a documentary about Cambodia, and also Princess Diana in Angola visiting mine-extraction operations. These two things triggered in me the magnitude of the problem.” He began to consider landmine-detection systems: In theory, what kind of engineering solution would work best? Then he met a Dutch researcher who had come across stalled plans to try to use cockroaches to detect TNT exuding from buried landmines. “I thought, yes—this was the way forward: using local resources, a solution based on what was available in the context. This was for me an a-ha moment.”