Usage Note
Insidioso describes something that causes harm gradually and deceptively, without obvious warning signs — commonly used of diseases (una enfermedad insidiosa), propaganda, or manipulative behaviour. The feminine form is insidiosa. The related noun insidia means 'treachery' or 'a trap'.
Examples
"El veneno actúa de forma insidiosa en el cuerpo."
Natural Translation
The poison acts insidiously in the body.

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