What is pokerus?

my pokemon havent got pokerus but i was just got curious... i know that pokerus is a good thing. it helps you train much faster, and with more ease.
but how long will pokerus last?
how can my pokemon get pokerus?
how do i know if my pokemon got pokerus?
you can add any extra details ( optional)
plz help?!!!
8 answers 8