>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 ~ Bulbapedia
The chances of one of your Pokémon contracting Pokérus are about 0.00457%. In other words, really low. In fact it's roughly 37% lower than encountering a Shiny. Over about seven years of playing Pokémon I have only encountered it twice.