>> how does evidence become tainted????????

... can come about in several different ways -- illegal search or seizure being the most common. In North's case, the appellate court found that the evidence used against him at trial had included his immunized testimony before Congress (that is, that he had testified before Congress under a grant of immunity from prosecution). The trial court should not have permitted the use of this evidence and, apparently, the prosecutors decided that without it they didn't have enough evidence to convict in a retrial.
----- jim o\-S