Nitric oxide (NO) possesses antiparasitic effects on both Protozoa and Metazoa in vertebrate definitive and intermediate hosts. Inducible NO limits parasite development also in Rhodnius prolixus and Anopheles stephensi, the natural vectors of human trypanosomiasis and malaria respectively, and in the snail Biomphalaria glabrata, a natural invertebrate intermediate host of human schistosomiasis. Therefore, NO limits Trypanosoma, Plasmodium, and Schistosoma development at all stages of the parasite life cycle.