Pokerus comes in different 'strains', these strains are calculated with 'X' and 'Y' values to determine how long it will last. You kind find the info here, but basically strain A lasts for 1 day, B lasts for 2, C lasts for 3, and D lasts for 4. People can hack their Pokemon to have a strain that does not exist in normal gameplay, so that the Pokerus will last longer. The fact that you got your Ursaring on Wonder Trade at lv. 100 makes it more plausible that someone just hacked it and decided to WT it away.