Insidious

Vocabulary word

insidious

adjective, uk /ɪnˈsɪd.i.əs/, us ɪnˈsɪd.i.əs/

(of something unpleasant or dangerous) gradually and secretly causing harm:

High blood pressure is an insidious condition which has few symptoms.

https://dictionary.cambridge.org/dictionary/english/insidious