The human parasite Leishmania is a probiotic for the fly that carries it

(BioMed Central) The Leishmania parasite, which causes the human disease leishmaniasis, acts as a probiotic in the insect that transmits it to humans, protecting them from bacterial disease. Findings published in the open-access journal Parasites and Vectors suggest that using bacterial controls to stop the spread of leishmaniasis could sometimes have the opposite effect to that intended, by benefiting flies carrying the parasite.
Source: EurekAlert! - Infectious and Emerging Diseases - Category: Infectious Diseases Source Type: news