Pokérus

Description
The Pokérus is a microscopic life-form that may attach to Pokémon, which your Kanto starting Pokémon will be infected with in Sinnoh. It is a beneficial mechanic that a Pokémon can obtain. Your chosen Kanto starting Pokémon will be infected by the Pokérus after clearing Distortion World for the first time. Once a Pokémon is infected by the Pokérus, it may gain Effort Values (EVs), which increase attack from that Pokémon.

Spreading the Pokérus
Spreading the Pokérus can only occur in the Pokémon Day Care. Currently, only a Pokémon infected with the Pokérus, and that's contagious, is able to spread it, and only manually breeding that Pokémon will allow the Pokérus to spread to other Pokémon with at least one type in common being manually hatched at the same time. Or in other words, put your infected Pokémon as well as another Pokémon or three with at least one type in common you wish to infect in the day care, and so long as your starter Pokémon isn't ready to hatch yet, all other Pokémon currently being hatched (manually, no helpers) will be infected by the Pokérus.

Effort Values (EVs)
EVs are a damage multiplying stat for each individual Pokémon that can currently only be seen in the Pokédex page for that Pokémon. EVs for a Pokémon can currently only be increased by catching that Pokémon again after it has caught the Pokérus. Gaining EVs can be done by catching a Pokémon on a Route, in a Dungeon, getting it as a Wanderer or via Evolution Items. Breeding does not count for gaining EVs. EV gain is affected by the Rowap Berry aura. See Farming Setups for an optimized EV gain farm setup

EVs Gain
Different methods of catching a Pokémon will give different amounts of EVs. If the Slow EVs Challenge Mode is activated, you gain 10x less EVs. ''Note: Shiny caught Pokémon gain 5x more EVs than listed below. Catching Pokémon with the Repeatball also gains 5x more EVs than listed below. I.E. Catching a shiny roaming pokemon with a repeatball offers 5 x 5 x 5 = 125 EVs''

EV Damage Multipler
EVs benefit the Player by increasing the amount of damage that a Pokémon does. EVs multiply damage done by up to 2x. If your EV is below 50, you gain 1% extra damage per EV. I.E. with 5 Effort Value, you have a 1.05x damage multiplier for that Pokémon. Pokemon with less than 50 EV are "Contagious", noted by this symbol in the Hatchery: Pokemon with 50 or more EV are "Cured", noted by this symbol in the Hatchery

EV Formulas
When Effort Value below 50: $$\colorbox{White}{$ \text{EV Damage Multiplier} = 1 + (\text{EV} / 100)$}$$ When Effort Value 50 or greater: $$\colorbox{White}{$ \text{EV Damage Multiplier} = 1 + \text{Min}(1, ((\text{EV} - 30)^{0.075} - 0.75 ))$}$$