Final answer:
In literature, 'insidious' typically refers to something subtly harmful. Good synonyms include 'stealthy,' 'sneaky,' or 'treacherous,' which imply a hidden danger that becomes gradually apparent.
Step-by-step explanation:
In the context of literature, insidious is often used to describe something that is slowly and subtly harmful or destructive, yet not immediately noticeable. A good synonym for insidious might be stealthy, sneaky, or treacherous. Each of these words carries the connotation of an underlying danger that is not obvious at first glance but gradually becomes more apparent as its effects spread.