Catching wild pokemon or breeding has a slight chance of getting a pokemon with pokerus. When a pokemon with pokerus gains experience in battle (thus, earning the respective EV's), the virus doubles the TOTAL EV gain from that battle.
i.e; battle a Woobat with Power Anklet equipped on your pokemon ---> 1 Speed EV from Woobat + 4 Speed EV from Anklet = 5 Speed EV ---> (5 Speed EV x 2) = 10 Speed EV from a single Woobat battle!
or
battle a Squirtle with Power Anklet equipped on your pokemon ---> 1 Defense EV from Squirtle + 4 Speed EV from Anklet ---> (1 Defense EV x 2) + (4 Speed EV x 2) = 2 Defense EV and 8 Speed EV from a single Squirtle battle.
As for getting pokerus, the best way would be asking someone in the Online Play section of the forums to trade you a crappy pokemon with the active virus (so you can spread it).
Pokerus spreads (chance) from an infected pokemon in your party to your other pokemon, every time you win a battle (there's no need for the infected one to participate at all; just being in your party is enough).
Infected pokemon IN YOUR PARTY, over time, will be "cured" and no longer spread the virus (but they'll still receive double EV even if they can't spread pokerus). This doesn't happen to infected pokemon in a PC box (the virus will remain forever active while it's stored in your PC), So my advice would be to spread your pokerus to 2-3 crappy pokemon that you'll never use (Rattata, Bidoof, etc) and store them in your PC, only taking them out when you need to spread pokerus to other pokemon. If one of those become "cured", just capture a new crappy one and infect it. Replace, rinse and repeat.