Once you see Ick on your fish, your entire aquarium is infected, Ick lives in the substrate and does not appear on fish till late in it's live cycle.
My local fish store (one of the P chains) is horrible about getting fish in that are infected with Ick so I have had some practice getting rid of it. From what I have read online Ick has a hard time surviving in waters that are over 80 degrees, so if you don't have any live stock or plants that are cold water dependent it is advisable to raise the temperature in your aquarium to 81 degrees. I have tried several types of medications to cure Ick and the one that works the best for me is QuICK Cure. Just follow the instructions on the bottle and you should be okay. I will often run a treatment for 2 weeks instead of the suggested 1 week treatment.