https://blogs.scientificamerican.com/observations/a-new-way-to-think-about-mental-illness/

Have you ever heard of a condition known as “general paresis of the insane”? Probably not. In the 19th century general paresis was one of the most commonly diagnosed mental disorders. Its symptoms included odd social behaviors, impaired judgment, depressed mood and difficulty concentrating. Around the turn of the 20th century, though, we figured what it really was—a form of late-stage syphilis infecting the brain and disrupting its function. A few decades later we discovered a highly effective treatment: penicillin.

Although general paresis is now very rare, its example is still instructive. Any honest researcher will tell you we don’t currently have good explanations for most mental disorders. Depression, obsessive-compulsive disorder, schizophrenia—we don’t really know how these patterns of disrupted thought, behavior and emotion develop or why they stick around.