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              insidious
From Latin insidisus, from insidiae - ambush (überfallen); from insidere - to sit upon, lie in wait for
1. Working or spreading harmfully in a subtle or stealthy manner: insidious rumors; an insidious disease. 
2. Intended to entrap; treacherous: insidious misinformation. 
3. Beguiling but harmful; alluring: insidious pleasures. 
Source: The American Heritage Dictionary of the English Language, Fourth Edition
There must be lots of insidiousness about - check out these synonyms:
artful, beguiling, corrupt, crafty, crooked, cunning, dangerous, deceitful, deceiving, deceptive, delusive, designing, dishonest, disingenuous, double-dealing, duplicitous, ensharing, fallacious, false, foxy, guileful, intriguing, knavish, lying, Machiavellian, mendacious, misleading,  perfidious, perilous, roguish, secret, slick, sly, smooth, sneaking, sneaky, stealthy, surreptitious, treacherous, tricky, two-faced, underhanded, untrustworthy, untruthful, wily, worm