From henceforth and retroactively, all purified Shadow Pokemon will now obtain or now currently have a non-spreadable version of Pokerus. The in-canon explanation is that the Shadow virus is based off of the Pokerus virus and while as a Shadow Pokemon they technically have Pokerus they can't benefit from it until purified which upon purification makes the Pokerus virus inert and thus no longer can spread like it does as the Shadow virus.

The benefits of the Pokerus virus are thus; a Pokemon with Pokerus earns twice the amount of Evo Points it normally would or if they use Happiness Points they earn an additional +0.5 every time they gain Happiness Points.

While you might be asking what benefit does a fully evolved Pokemon receive when having Pokerus the short version is it doesn't. However, if you have a Destiny Stone there is a benefit insofar that the points earned for the stone's stockpile are doubled thanks to the Pokerus. We are also working on ideas for items such as Exp Share and Exp All which could also benefit other Pokemon who do not have Pokerus.

That's all for now, later~