Because of their weakened immune systems, people with HIV develop TB when infected with far fewer bacteria than are needed to make a healthy person sick. Relatively few bacteria in the cough-and-spit sample make it less likely that a technician using a microscope will spot them. But they still produce a smell that can be detected by the rats.

It may even be the case that the rats can sniff out the bacteria at levels so low as to be undetectable even by sophisticated laboratory techniques, Mgode suggested. “You will get a [clinic] sample that is indicated by 11 rats, but you can’t confirm it’s TB [with the LED microscopy]. To me …” he tapped his chest, “…to me, I know this is TB. But since we don’t have a conventional method which is approved, we don’t report this patient.”

The rats have screened 342,341 samples. Overall, they have hiked the TB case detection rate by around 40%

If the rats can detect TB at an earlier stage of infection than any other method, this could be a huge benefit, since a patient who is treated earlier is less likely to transmit his or her infection to other people. Mgode is planning to do more research to investigate this. But the biggest aims right now with the TB programme are more practical: getting results to patients faster and expanding the programme’s reach.

It's a strange form of medical practice to have a rat sniff you for TB.