Trichoplax, one of the simplest animals on Earth, lives in a highly specific and intimate symbiosis with two types of bacteria. One, Grellia, is related to parasitic bacteria that cause typhus and Rocky Mountain spotted fever. However, it does not appear to harm Trichoplax. The other, Ruthmannia, sits inside the cells Trichoplax uses to digest its food. This symbiosis provides a window into microbial dark matter.