Insidious is a term used to refer to the state of a potentially harmful activity or substance increasing in quantity or prevalence at a gradual pace. It describes the slow spread of infectious diseases, behaviour or activities within a population.
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what does insidious mean
intended to entrap or beguile: an insidious plan.
stealthily treacherous or deceitful: an insidious enemy.
operating or proceeding in an inconspicuous or seemingly harmless way but actually with grave effect: an insidious disease.