Pokerus
Pokerus is a virus. That’s how it gets the name, Pokemon and Virus: Pokerus.
However, just like there aren't any animals in the Pokeworld, there also aren't any animal viruses. So, we can assume that Pokerus is actually a Pokemon too.
The Pokerus, however, like most Pokemon are very social, they travel in groups.
Being so small, Pokerus cannot attack like any other Pokemon. Can you imagine something smaller than a speck of dust trying to electrocute a Wailord?
No, Pokerus are very devious. They attack from inside the Pokemon.
And the Pokemon are just as devious. They have developed an antibody that is passed down genetically. When Pokerus strikes, it activates strengthening the Pokemon's senses, muscles, and thought process, to counteract the Pokerus's attacks.
And, since nothing is perfect, the antibodies don't stop working after the Pokerus has been sufficiently counteracted. This causes the Pokemon's stats to grow higher faster than a Pokemon whose antibodies haven't been called into play.
Or, because the continued action of the antibodies is beneficial to the pokemon, they have developed genetically to allow for this.