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.