You mean the very first person in the history of the world who caught it? We don't know. It's a virus that, at some point, mutated and one unlucky person ingested that first virus, or it mutated while in that person's digestive tract. Since then, it has been successful enough in its virulence to continue infecting people, and enough of the virus hasn't mutated into being non-infectious so that it still causes noticeable outbreaks. It happens very frequently, viruses mutate, and some strains die out while new ones take over. It's the reason why there are different strains of the flu, they keep mutating. Sometimes they mutate into harmless viruses, sometimes they mutate into more brutal ones.