What’s the difference between insidious and invidious, and what about perfidious and pernicious, for that matter? None of the four words is synonymous with any of the others, though your connotation radar may correctly sense that they all have unpleasant associations.
Insidious, which derives from the Latin word for “ambush” (the second syllable is cognate with sit), means “treacherous” or “seductive,” with an additional connotation of “subtle,” in the sense of a gradual, cumulative effect. (This, unlike the other meanings, is neutral, but the word is rarely used except in a negative sense.) For example, in medical terminology, an insidious disease is one that remains hidden until it is well established. The noun form is insidiousness, and the adverbial form is insidiously.
Invidious, meanwhile, which stems from the Latin word for envy, refers to feelings of animosity, discontent, or resentment, or to obnoxious or even harmful behavior.
Perfidious (the second syllable of this word is cognate with fid- in fidelity) means “treacherous” or “disloyal”; the noun form is perfidy. Pernicious, meanwhile, means deadly. (The second syllable is cognate with nox- in noxious.) Pernicious anemia is a particularly serious form of blood-cell depletion that might as well be called insidious anemia because of its slow onset, and pernicious scale, also known as San Jose scale (for its discovery in the California city of that name), is an insect that infests and kills trees.
The noun and adverbial forms of invidious, perfidious, and pernicious follow the same pattern as those for insidious.