What is Pokerus?
Pokerus is a condition that can affect all types of Pokemon. It is a beneficial condition that doubles your effort value (EV) gained after each battle. This condition is very rare and only has a probability of 1 in 21,845, therefore having a higher chance of encountering a shiny Pokemon.
You can see if your Pokemon has Pokerus if it has a violet status symbol with Pokerus written on it.
Infection and Cure
Pokerus is highly contagious and can spread in your party after a battle. The Pokemon adjacent to the affected Pokemon can contract it after a fight. However, a Pokemon can never be infected twice. Pokemon Eggs can also catch the condition.
The condition is not permanent and will disappear in 1-4 days. The Pokemon has a chance to be cured every midnight. His status symbol will be replaced with a special smiley face, indicating that he was once a carrier of Pokerus. Also, The double EV gain does not disappear after its duration.
Keeping the infected Pokemon in the PC box delays the healing process and can give you more time in infecting more Pokemon in your party. Since this condition greatly increase the power of your Pokemon, it is highly advisable that you spread it to all your Pokemon.
Trading Pokemon with your friends and other trainers using Global Trade Station (GTS) is a great method as it can spare you the hardships of finding an infected Pokemon in the wild.