RR2 explains that a fairly large percentage of people who have suffered the Slarecian Language Virus get well after a few weeks... but when they recover, they now have the potential for psionic powers. The book suggests that this was probably not the intent of the virus, and is in fact a side effect caused by a mutation in the virus. (Or it may all be part of a plot to return the Slarecians to the world by increasing the psionic energy levels there.... )