The genophage is a fictional biological weapon from the video game series Mass Effect. It was created by the Salarians and used against the Krogan species to control their population. The genophage causes infertility in the Krogan, significantly reducing their birth rates and preventing them from overwhelming the galaxy. The genophage was developed in response to the Krogan's rapid expansion and aggressive behavior, which posed a threat to other species. Its effects have led to a long-standing conflict between the Krogan and the Galactic Community, raising ethical questions about the use of such a weapon in warfare.