I am just wondering how does a Pokemon get pokerus?

A couple things. 1st there is no easy way to get pokerus, getting it is comparative to finding a shiny pokemon. It is very rare and totally random. 2nd and most important from reading your question is that pokerus does NOT make your Pokemon level up faster. It does not effect the amount of exp obtained at the end of a battle. What it does is doubles the EVs obtained at the end of a battle. Judging by you trying to a Pokemon to level 100, it's probably already past the point of obtaining EVs and get pokerus on it would be absolutely no good other then to infect other lower level pokemon. If you don't know what EVs are please go look through some of the questions and look for one dealing with them.
P.S If you are trying to raise it's levels faster, try to find a lucky egg off a wild chansey.

Well, you cannot really 'get' the Pokerus virus you see. The odds of you actually getting Pokerus are like, slim to none. You cannot really look for it, the virus will just decide to come and infect your Pokemon. It could take could take hours, days, and weeks on end before your Pokemon gets infected with Pokerus. The best thing to do is to just continue playing the game, and do not worry much about getting infected with Pokerus. While infected with the Pokerus, your Pokemon's Effort Value points will double in size whenever your Pokemon grows a level. That means that they get double the amount of stats when they go up one level. In order to spread the Pokerus to other Pokemon so that they can get infected as well, you should stick the infected Pokemon in the PC for a while. Soon enough, some Pokemon should be infected by Pokerus.

Hope this helps.