What EXACTLY does Pokerus do?

Question

I know its good for training, but what is it exactly? What does it do?

Answer 1

PokéRus basically doubles the amount of EVs obtained but lasts for only a certain amount of time. Pokerus is NOT dangerous.

And it appears randomly, so if one of your gets it, put it in your box so it will keep it. To spread it just put the Pokemon with pokerus in your party and put other Pokemon that have never had it before in with it, and complete some battles. When a Pokemon is cured from pokerus you will see a little smiley next to the Pokemon's name. Once a Pokemon gets cured from pokerus it will never get it again

Hope I helped!

Answer 2

It doubles the EVs, or Effort Values, that you gain. Note that this does not include EVs gained by Super Training. Just in case you don't know what EVs are, I'll explain.

Effort Values are attributes which give bonuses to an individual Pokemon's stats and improve differently depending on what Pokemon they defeat.
This means that each Pokemon gives different Effort Values, generally based on their highest stat. Geodude give off Defense Effort Values. It takes 4 EVs to increase a stat by one point, and a Pokemon caps at 252 EVs in one stat, and 510 in total.

For example, let's say I have a level 1 Torchic who I want to have a high Attack stat. I look up which Pokemon give Attack EVs, an example being Bellsprout. For every 4 Bellsprout my Torchic defeats, it gains 1 Attack point.
Pokerus doubles this quantity. If my Torchic has Pokerus, it will gain 8 Attack EVs instead of four, making its Attack stat go up by 2 points rather than 1 after defeating 4 Bellsprout. Note that the EV caps are still the same.