Macho Brace
>Effect: Doubles any effort values the Pokémon holding it gets, but halves its Speed in battle. Stacks with Pokérus.
Power Weight
>Effect: Promotes HP gain on level up, but halves Speed stat in battle. Adds 4 HP EVs whenever experience is gained in battle, regardless of the Pokémon fought. Its effect is doubled by Pokérus.
Power Bracer
>Effect: Promotes Attack gain on level up, but halves Speed stat in battle. Adds 4 Attack EVs whenever experience is gained in battle, regardless of the Pokémon fought. Its effect is doubled by Pokérus.
Power Belt
>Effect: Promotes Defense gain on level up, but halves Speed stat in battle. Adds 4 Defense EVs whenever experience is gained in battle, regardless of the Pokémon fought. Its effect is doubled by Pokérus.
Power Lens
>Effect: Promotes Special Attack gain on level up, but halves Speed stat in battle. Adds 4 Special Attack EVs whenever experience is gained in battle, regardless of the Pokémon fought. Its effect is doubled by Pokérus.
Power Band
>Effect: Promotes Special Defense gain on level up, but halves Speed stat in battle. Adds 4 Special Defense EVs whenever experience is gained in battle, regardless of the Pokémon fought. Its effect is doubled by Pokérus.
Power Anklet
>Effect: Promotes Speed gain on level up, but halves Speed stat in battle. Adds 4 Speed EVs whenever experience is gained in battle, regardless of the Pokémon fought. Its effect is doubled by Pokérus.
Straight from Bulbapedia, the all knowing.
Also, a bit about Pokerus, that has been mentioned so many times:
>When a Pokémon has the Pokérus, it gains double the effort values from battling (e.g., fighting a Magikarp will give two Speed EVs, rather than one). Effort points gained from vitamins and wings are not doubled. The infected Pokémon can infect other Pokémon with the virus for a period of about two days. However, the Pokérus timer can be delayed by several methods including placing the infected Pokémon in a PC Box.
While it is represented similarly to status conditions, the Pokérus is not a status condition, so it cannot be healed at a Pokémon Center or with any status condition healing item.
Pokérus has a 3 in 65,536 or approximately a 1 in 21,845 chance of being generated on a Pokémon, either wild or bred, effectively making this rarer than a shiny Pokémon, which have a 3 in 24,576, or around 1 in 8,192 chance to be found wild or hatched. Through battling or capturing this wild Pokémon, players could get this helpful virus onto their own Pokémon, and subsequently spread it around. Due to the overwhelmingly low chance of encountering the virus (about 1/3 of the chance to encounter a Shiny Pokémon), and the fact that most would not even recognize a Pokémon with the virus and may flee from it, many players don't have a Pokémon with the virus.
tl;dr: Something so rare it probably doesn't matter anyway, but keep a look out for it, and if at you have the good fortune, then make full use of it.
Again from Bulbapedia, I'm such a leech (but a credit giving one, lulz)