Nematocida parisii, a newly discovered species of the protozoan parasites known as microsporidia, lives in the intestines of C. elegans, a small roundworm commonly used for research.

Microsporidia can be found in water supplies and can cause a variety of infections, known as microsporidiosis, that affect people with weakened immune systems. The parasites can infect the lungs, kidney, brain, muscles, eyes, sinuses, or intestines, and can cause gallbladder disease and a debilitating array of gastrointestinal symptoms, including chronic diarrhea and wasting.

This image shows a section of roundworm intestine stained to show N. parisii's cytoplasm in red and DNA in blue.