Genophage

The genophage is a biological weapon in Mass Effect, deployed against the krogan by the turians during the Krogan Rebellions to severely reduce krogan numbers by infecting the species with a genetic mutation. The salarians developed it as a deterrent they believed no one would ever use, but the turians released it as soon as it was complete. Rather than reducing female fertility, it cuts the chance of a viable pregnancy, so that most fetuses never develop, and because every cell of every krogan is infected, gene therapy cannot counteract it. Together with krogan disunity and violence, it has left them a dying race.