Genophage

The Genophage was a biological weapon used against the Krogans by the Turians. It was designed to reduce the Krogran race by making only one in one thousand births stay alive. Because of this the Krogan are a dieing race soon to be extinct.

When the Krogans were breading at an incredible rate due to the relocation of their society away from a harsh planet. They spread onto other planets as a consequence of this survival mechanism over the course of three centuries the council decided that action needed to be taken in order to stop any number of impending disasters due to over population. The council contacted the Turians and asked them to design a weapon to reduce the Krogan population which resulted in Genophage.