Each year, more than a million people in tropical countries a contract leishmaniasis, a parasitic disease spread by female sand flies. After the insect delivers an infectious bite, there are several possible outcomes for the human host, including disfiguration and death.

For the fly, however, the experience is quite different.

According to research recently published in the journal Parasites and Vectors, the Leishmania parasite affords infected sand flies some immunity against bacterial disease. In other words, what humans experience as a pathogenic invader acts as a probiotic protector for flies — a discovery with potential ramifications for fighting the spread of leishmaniasis.

Rod Dillon, a senior lecturer in biomedical and life sciences at Lancaster University in England, and his colleagues raised sand flies in their laboratory and infected more than 1,000 of them with Leishmania parasites. Then they fed infected and uninfected flies a concoction of blood and sugar or blood and Serratia, a sand fly bacterial pathogen. Six days after eating the pathogen, five times as many of the infected flies were still living, compared with those that did not harbor the parasite.

Previously, researchers discussed using Serratia as a biological control for sand flies, but these new results imply that strategy would not be an effective method of slowing the disease’s spread. “You would end up with just flies with leishmania,” Dr. Dillon said.