The insidious disease seemed like just a harmless cold at first, but then killed its victims in the end.
I know we are working against an enemy who is very insidious in his methods.
Insidious definition:appealing but imperceptibly harmful, seductivesentence: Liza's insidious blueberry muffins taste so good but makes you fell so sick later on!
The book's summary revealed the main character's insidious plot to obtain the large inheritance, illegally.
The most insidious danger may be more abstract.