When I caught thundurus, I was lucky enough for him to be perfect and have pokerus 1/65,235 chance of a pokemon with pokerus
When you say perfect, do you mean perfect IVs? That'd be quite something, given that the chance is 1/1048576. Meaning that the chance of a perfect IV pokerus thundurus is 1/68,403,860,000. That'd be quite something indeed.