Difference between 'Insidious' and 'Pernicious'
I'll go again with Merriam-Webster 3rd International:
insidious:
- watching for an opportunity to ensnare: insidious tempter
- intended to entrap or trick: insidious plot
- acting by imperceptible degrees: insidious disease
- having gradual, cumulative, and often hidden effects; insidious pressures of modern life
- subtle: insidious charm
For synonyms it gives 'sly', but obviously that won't work everywhere, mostly 1, 2, possibly 5.
Pernicious has none of these meanings. It basically means destructive or harmful. in the names of diseases (pernicious malaria, pernicious anemia) it means especially severe forms of the disease.
Insidious suggests a lying in wait or a gradualness of effect or approach and applies especially to devious and carefully masked underhandedness.
- a part of an insidious conspiracy to undermine the world- Edmund Wilson
- an insidious tempter
Pernicious is more often applied to things that harm exceedingly or irrepairably by evil or by insidious corrupting.
- a pernicious influence
- pernicious propaganda
(excerpts from MW dictionary of synonyms)
The actual OED does not make them synonyms, defining "pernicious" as extremely harmful (from the Latin per (intensive prefix) + nex, necis death) and "insidious" as treacherous (from the Latin insidere, to lie in wait). Ambushers lying in wait for you can't intend anything but harm, but they needn't mean to kill you. And something can be pernicious and readily apparent. Consider the sentence from The Province of Jurisprudence by John Austin:
What appears pernicious to one person may appear beneficial to another.
That doesn't mean that the two can't go together. Pernicious anemia was named before its cause was understood, at a time when it was invariably fatal. But it is also an insidious disease, developing slowly with multiple symptoms that evade an easy diagnosis.