the blues; a fit of depression
Idiomatic expression referring to a gloomy mood, based on the folk belief that a 'bug' inside the body causes depression. Often used in phrases like ふさぎの虫が騒ぐ or ふさぎの虫がつく.
See also: 虫
Lately I've been feeling the blues and can't get motivated to do anything.
She's sunk in gloom as if possessed by a depression bug.