Ophryocystis elektroscirrha is a protozoan parasite that infects monarch and queen butterflies. Spores of this parasite are ingested by the caterpillars and will migrate through their bodies to the hypoderm. When the caterpillar pupates, the parasites reproduce sexually and produce lots of daughter spores. These spores will cover the scales and abdomen of the adult butterflies when they emerge, where they will be ready to be rubbed off onto the host milkweed plants and available for the next generation of monarchs to ingest them and begin the cycle all over again. The fitness effects of this parasite on the butterflies is varied and includes lower survival, smaller size, and lowered flight performance.
Image is from The Monach Lab at the University of Minnesota.