Dog noses are 10,000 to 100,000 times as sensitive as human noses. Scientists are not sure exactly what dogs are smelling, but it is known that malaria parasites produce volatile aldehydes like those found in perfumes.

The parasites may have evolved the ability to exude odoriferous chemicals in order to attract mosquitoes to carry them to new hosts. Studies have shown that mosquitoes prefer to bite people who have malaria.

If just one chemical indicated cancer or malaria, “we’d have discovered it by now,” said Claire Guest , who founded Medical Detection Dogs in 2008 and oversaw dog training in the study. “It’s more like a tune of many notes, and the dogs can pick it up.”

Most breeds have good noses, she said, but the best for this task are dogs bred to hunt — like pointers, spaniels and Labradors — and dogs with relaxed relationships with their owners.

The initial trials were just to prove that detection was feasible, said Steve W. Lindsay , an entomologist at Durham University in Britain who said he was inspired by a dog sniffing luggage for contraband food at Washington Dulles airport.

This preliminary study involved training just two dogs to sniff rows of jars containing bits of thin nylon socks that had been worn overnight by Gambian children.

When the dogs, a Labrador-golden retriever mix named Lexi and a Labrador named Sally, recognized the telltale odors, they were supposed to stop and point at the jar.