PKRS stands for "Pokerus" and it is a very rare infection which your Pokemon can have. Don't worry though, Pokerus is actually beneficial. If you're familiar with EV training, then you know that KO'ing a Pokemon and receiving EXP will get you EV point(s) in a particular stat(s). KO'ing a Magikarp normally gives you 1 Speed EV, for example.
Pokerus doubles any EVs you gain. If you had Pokerus and KO'd that same Magikarp, you would now gain 2 Speed EVs instead of one. If you're not into competitive battling, then don't worry about Pokerus for now.
EDIT - I would suggest that you infect another poke with Pokerus and keep that poke in your box at all times. When you see the purple PKRS on the Summary screen, it means that you have the "infectious" state of the disease (meaning you can infect other pokes). Just stick the poke with Pokerus in the 1st slot and the poke you want to infect into the 2nd and run away from a few battles. The poke in the 2nd slot will eventually receive Pokerus. You want to stick it in the box as all pokes with the "infectious" stage of Pokerus will be cured when the clock rolls over to midnight (IIRC). Once cured, that poke will still get double EVs but will no longer be able to infect other pokes.
