Quality control is a first step, and a large one. Medical scent detection is more complicated than drug or bomb detection, Dr. Otto said. A dog working an airport for drugs or explosive detection has a consistent context and a fairly straightforward target odor. In Covid detection, researchers know that the dogs can distinguish an infected person’s sweat or urine. But they don’t know what chemicals the dog is identifying. Because human scents vary, medical detection dogs have to be trained on many different people. “We have all of the ethnicities and ages and diets and all of these things that make human smell,” Dr. Otto said. The symptoms of many medical conditions are similar to those of Covid, and dogs that detect scents associated with fever or pneumonia would be ineffective. So the human subjects used in training dogs, Dr. Otto said, must include “lots of people that are negative, but might have a cough or might have a fever or other things.” If the dogs mistook flu for Covid, that would obviously be a crucial mistake.