The Genophage is a biological weapon that features in Mass Effect.
The Genophage was an insidious bioweapon developed over a thousand years ago by Salarian scientists during the turbulent and violent period of the Krogan Rebellions. At the time, the aggressive Krogan who had previously been loyal Citadel followers had been reproducing at an astonishing rate and threatened to overwhelm their allies. This led to the Salarians and their fellow Citadel members in finding a means of diminishing this threat. The first act was the creation of a genetic weapon keyed to Krogan DNA designed to target them and reduce their reproductive capabilities as well as their natural talents. The second act as the admission of the Turians into the Citadel who used the Genophage and deployed it against the Krogans. In time, the entire species was affected and their numbers gradually diminished leading to an end to the threat posed by the Krogans. Whilst the race still existed, they were but a shadow of their former selves and the threat of war against them had been eliminated. Many Krogans and outsiders had the mistaken belief that the race was rendered sterile but in fact their natural biological abilities had been greatly suppressed in order to make them more manageable.
In the years that followed the Systems Alliance's entry into the Citadel, the Salarians had been using their Special Tactics Group to study the Genophage. Whilst seemingly a benign act, an ulterior motive was present as the teams consisting of scientists were tasked with studying its continued effects. Prominent Salarian scientists such as Mordin Solo came to a startling conclusion which was that the Krogan's adaptive biology was coping with the Genophage and rapidly overcoming its effects. It was believed that, in time, the Krogan would return to their full potential. This led to the order to create a new variation of the Genophage in order to reinforce its effects and was done so in a covert basis. Mordin Solo believed that this was a prudent course of action and was the right choice to make as there was a threat of renewed Krogan Rebellions if the Genophage was overcome.
The Genophage was a weapon employed by the Turians and Salarians for use against the Krogans during the Krogan Rebellions. The weapon resulted in the Krogans facing near sterility with fewer members of the species being born per generation.