For plastic (and buckets especially), once infected, always infected IMO. You can try to get by with a long sanitizer soak, but don't plan on those beers staying clean.
Speaking of - if a prior batch was infected, and you took samples or transferred it with your "clean" gear, that may be where the infection is coming from. When did you notice the infection?
My guess is that, unless you're doing a lot of open fermentation, its not airborne and you're using an infected piece of equipment on the cold side.
Easily mixed-up soft parts: Bucket lids (and gaskets), airlocks/blowoff tubes, thief, plastic covers for racking cane, kettle valve/pump, plastic stir spoon, oxygen stone (plastic connection tubing and/or worm clamps that may harbor bugs).