The genophage was a biological weapon deployed against the krogan by the turians during the Krogan Rebellions. It was designed to severely reduce krogan numbers by "infecting" the species with a genetic mutation. The genophage's modus operandi is not to reduce the fertility of krogan females, but rather the probability of viable pregnancies: many krogan die in stillbirth, with most fetuses never even reaching this stage of development. Moreover, every cell in each krogan is infected, to prevent the use of gene therapy to counteract it.
The genophage was originally developed by the salarians. They believed that, as the consequences would be so devastating, no one would ever deploy the genophage, thus wielding it as a deterrent. However, the turians had different military attitudes, the turians only appreciating an approach of "massive retaliation". Once the genophage was complete, the turians immediately used it, and krogan numbers began to dwindle rapidly. Any female krogan who could carry young to term became a prize of war who was viciously fought over. Taking advantage of this, a female warlord named Shiagur used her fertility as a bargaining chip to attract the strongest males to her army.
A few individuals, such as Wrex, have attempted to rebuild the species, or at least curtail its decline, by focusing on breeding, but the inherent belligerence of the krogan usually frustrates their efforts.
There is no known cure for the genophage. Some krogan groups have invested in companies such as Binary Helix to try to discover one; however, efforts have so far been thwarted by a lack of krogan scientists and a disinterest of other species in a revived krogan population. At one point, the Krogans started to evolve a resistance against the genophage. The SSTG developed a second, modified genophage to deal with this.
The krogan are a shadow of their former glory. While the Rebellions took place centuries ago, they are constantly reminded of the horror of the genophage and of their inability to counter it. The release of the genophage is still controversial, bitterly debated in many circles.
Mordin Solus sheds light on the genophage he helped create, insisting that its purpose is not to wipe out the Krogans but to stabilize their large growth. He points out that during the development of the genophage, it was very possible to have made it so that it would wipe them out; he did not choose that path.