Observations on host-parasite-pathogen interrelationships involving protozoa are briefly reviewed. Discussion is centered on the role of insect parasites in the mechanical transmission of protozoa, the development of the parasite in protozoan-infected hosts, transovarian transmission of the host’s protozoan by the parasite, and the traceability of protozoan infection in most parasites to infection of the host.

The possible complexities of host-parasite-pathogen interrelationships involving protozoa are illustrated by the microsporidoses of the lepidopterous species, Heliothis sea (Boddie) and H. virescens (F.); the primary parasites, Campoletis sonorensis (Cameron) and Cardiochiles nigriceps Viereck; the hyperparasites, Catolaccus aeneoviridis (Girault) and Spilochalis side (Walker); and the microsporidans, Nosema heliothidis Lutz and Splendor, N. campoletidis Brooks and Cranford, and N. cardiochilis Brooks and Cranford.