The genophage bioweapon was created to end the Krogan Rebellions. The turians fought the krogan to a standstill, but the sheer weight of krogan numbers indicated they could not be stopped through conventional means. The turians collaborated with the salarians to engineer a genetic counter to the krogan's rapid breeding.
The genophage virus replicated by 'eating' key genetic sequences, altering every cell of krogan physiology so the krogan could not use gene therapy to fix the affected tissues. Once a genophage strain could replicate no more, it would starve and die, limiting mutation and contamination. In addition, the 'created' genetic flaw is hereditary. The resulting mutation made only one in a thousand krogan pregnancies carry to term, reducing offspring viability rather than fertility. Krogan warlords fought battles over the females able to carry children to term.
The release of the genophage is still controversial and bitterly debated in many circles.