insidious

insidious adjective uk/ɪnˈsɪdiəs/ us
having harmful effects that happen gradually so you do not notice them for a long time

постепенно наносящий вред, коварный
the insidious effects of pollution insidious ly adverb

исподволь
insidious ness noun [ U ]

коварство (болезни и т. д.)