how rare is pokerus
Encountering a Shiny Legendary or a random shiny Tentacool have the same rate of 1 in 4,096. However in the wild the chance of a Shiny Pokemon with Pokerus is 1 in 178956971. Your Latias is very rare as it is Shiny but also has Pokerus. However the Latias probably had Pokerus infected onto it via another Pokemon. There is also a possibility it was hacked too.