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 foetuses 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. Though the genophage was not designed as a “sterility plague”, the combination of a low frequency of viable pregnancies with the krogan proclivity to violence and indifference about focused breeding leaves the krogan a dying race, and soon to be extinct.
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 salarians and 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.
After the Reaper War the Krogans are still suffering from the devastating effects of the Salarian made genophage. Because of this, tempers have risen between the Krogans and the Turians, while the Salarians are as always held responsible as it was their minds who engineered the mutation. The Krogans are breaking up into factions of those, who violently oppose the genophage and will stop at nothing until there is a cure, and others who want to through diplomacy and good will prove that they can be leashed thus proving that the genophage is no longer necessary. Some of the Krogan from the prior faction are becoming openly hostile towards citadel space, and the Turian are under the persuasion that in order for Citadel space to be safe the Krogans must be eliminated once and for all or at the very least kept under a form of house arrest in their home solar system.