There is a 3 in 65,536 (about 1/3 of the chance to encounter a Shiny Pokémon) of enountering it on a wild or newly hatched pokemon.
I'm not exactly sure what you mean by 'from the PC', but the pokevirus will not spread in the pc and pokemon in the pc will not loose it. Please clarify if this was not sufficient.
Source: Bulbapedia