Documented human cases of a disease called toxoplasmosis go back as far as ancient Egypt. The condition is caused by a parasite that can only reproduce in the intestines of cats. Today about one-third of the world’s population is infected with toxoplamosis, but most people who have it will never know it.

Emory evolutionary biologist Jaap de Roode, whose research focuses on the co-evolution of parasites and their hosts, worked with animators and writers at TED-ED to create the video lesson, above. Watch it to get a brief overview of what is known about toxoplasmosis, and some of the big questions that remain.