The same way they select a new pope, they select a new disease.
(media.scored.co)
Nature will correct either way, a disease doesnt *want* to be deadly, it wants to be as infective as possible, as diseases that kill of their hosts too fast will die out. Its the reason why we havent had a world wide ebola plague, because with *any* amount of hygiene it only kills off its first few hosts and then dies.