Pokémon

A carrier of the virus found themselves in a world of magical creatures known as Pokémon. He wandered around the place for a bit before getting attacked by a wild Paras.

The Paras carried the virus and transferred it to a Pidgey, who transferred it to a Magikarp who transferred it to a fisherman. The virus laid dormant within them, getting transferred to more and more people and Pokémon before finally taking effect.