The sterility plague was a type of virus that spread throughout the Allied Sovereign Nations that rendered Vaikan unable to lay eggs. It was brought by visitors from the First Drussiray Empire on accident after an expedition to the Omni homeworld of Kollha on the other side of the galaxy. The Drussiray, due to their physiology, were immune, and they were not even aware they were carrying the virus. By the time they met with the Vaikan on Jaran, it was too late. The virus quickly spread across the entirety of Vaikan territory. Only the colonies that had completely isolated themselves were unaffected.
The virus persisted for several centuries meaning that the Vaikan had to rely on imperfect clones and extending their own life spans in order to avoid extinction. Vaikan population was already unstable due to a proportionally greater number of males to females. The Vaikan were still recovering from the Sinister Hegemony's genocide of their females which occurred thousands of years prior.
Over the course of several centuries, a cure was eventually developed and slowly distributed. However, the societal and psychological damage was already done. For most Vaikan that had gotten used to cloning, the idea of intercourse for the sake of reproduction was considered a societal taboo. A sizeable portion of the populace even refused to become cured after getting used to the life style of mass cloning.
The virus targets the reproductive organs decreasing libido and dampening the production of testosterone and estrogen. As a result, those who were cloned from a Vaikan infected with the virus would have a mostly androgynous appearance. Sexual dimorphism virtually vanished from the Vaikan for at least a few centuries.
Research indicates the virus was artificially created, and it may have been used by the Omni as a means of subjugating or culling their own population. The Omni were known to be rapid breeders laying eggs in clutches of over five hundred.
Why the virus only affects Vaikan is highly debated, but some theorize that it is because the Vaikan and the Omni share similar protein structures. It is widely theorized that this was a result of the Omni performing genetic experiments on prehistoric Vaikan and meddling with Ucharpli's ecological evolution.