Ah... well, first, there were people turning up who had contracted an unknown illness of some sort. Then, the pokemon centers in three cities were attacked, and shortly after that, people were falling ill all over the place. But a few days after the onset, the symptoms cleared, leaving those who'd been stricken with it with various physical changes to varying degrees, as well as certain abilities similar to pokemon. After about a week, there was another bout of illness and those abilities faded, but the physical changes have remained.