Exactly, but like I said, with both virus and skeletons, a potential loop could be created. Which is why unupped can arguably be better than upped. It becomes a power problem when combined with bone wall, condors, multiple boneyards, or multiple dead marshes.
Upped, it's basically just unlimited plagues (retroviruses will form again when the enemy's poisoned creatures die), but it's not more powerful than currently existing CC cards. And immortality will counter it hard.