The Nebari Contagion is a sexually transmitted disease which was given to an undisclosed number of non-conformist Nebari, including Chiana and her brother Nerri, by "The Establishment". These carriers were then allowed/encouraged to flee the Nebari territories.
Being that both Chiana and her brother were young and promiscuous, as we can assume were many of the non-conformist Nebari, they spread the disease throughout the galaxy quickly.
The purpose of the disease was that on a specified date, set by "The Establishment", everyone infected by the disease would be mind cleansed, though whole civilisations were unlikely to fall the Nebari Establishment hoped that enough of a given worlds populations would be incapacitated to allow the Nebari "emissaries" to easily secure affected territories.
Nerri was able to find a cure from a contact high up within the Nebari government which he shared with Chiana to stop them spreading the disease, the cure requires the endurance of great pain. ("A Clockwork Nebari")
The contagion is referenced by the Kkore, who are less than surprised that the Nebari have yet to proceed with their plan for biowarfare on a galactic scale.