Contaminated bitcoin is not a rare case. This happens very often. And of course, we always have a solution for that, so many systems can handle your concerns. Right on this forum, bitmixers and bitblenders are the two most reliable mixing systems. Try it, and you will get the best results. Don't hesitate!
In fact, many tainted coins were spread to others by the "mixers". I guess we do not need to worry about this for the time being. The exchanges (includeing bitcoin <--> Altcoin exchanges, such as Poloniex) are doing their best to avoid illegal activities.