in·sid·i·ous 
/ɪnˈsɪdijəs/ adjective
/ɪnˈsɪdijəs/ adjective [more insidious; most insidious] formal : causing harm in a way that is gradual or not easily noticed
▪ Most people with this insidious disease have no idea that they are infected. ▪ an insidious enemy
▪ Most people with this insidious disease have no idea that they are infected. ▪ an insidious enemy
— in·sid·i·ous·ly adverb
▪ The disease progresses insidiously.
▪ The disease progresses insidiously.
— in·sid·i·ous·ness noun [noncount]
