【Contaminate】 , 【taint】 , 【attaint】 , 【pollute】 , 【defile】 mean to debase by making impure or unclean.

【Contaminate】 implies the presence or the influence of something external which by entering into or by coming in contact with a thing destroys or may destroy the latter's purity.

【Taint】 differs from 【contaminate】 in stressing the effect rather than the cause; something contaminated has been touched by or mixed with what will debase or corrupt; whatever is tainted is no longer pure, clean, unspoiled, or wholesome but is in some measure or degree sullied or stained or in process of corruption or decay.

The less common 【attaint】 may be closely synonymous with  【taint】 . More often it retains a hint of its primary meaning of to sentence to outlawry or death and then suggests a sullying (as of one's name) or a degrading especially as a result of actual or reputed misconduct.

【Pollute】 implies that the process which begins with contamination is complete and manifest and that what was pure and clean has lost its clearness or fairness and has become muddy or filthy or poisoned.

【Pollute】  is especially apt when the reference is to something that ideally is clean, clear, or bright.

【Defile】 strongly implies befouling of something which ought to be kept clean and pure or held sacred. It usually suggests violation, profanation, or desecration and is highly opprobrious in its connotations.