I prefer disorder to disease.

Abuse in childhood certainly creates mental illness. I suffer with depression caused by my mother who was totally self absorbed (narcissist) A 'refrigerator mother'. I had a miserable, cold childhood.
Some mental illness is (I believe) genetic, so even if your parents are good and your childhood happy you could still suffer with it.

Like I get very annoyed when people insist my mother must have been abused as a child because mother is a malignant narcissist . But that's not the case at all. My mother's mother was a lovely woman, kind, generous and soft hearted. Everyone who knew her describes her as such especially my mother who remembers a happy childhood, with lots with fond memories.

Her father was a strange difficult man however, mean with emotion, money, food, and very controlling of those things.
He was not overtly abusive, it was the 1940s so he went to work and left childcare to his wife.
My mother is very very controlling and mean like her father. And no I absolutely do not believe she was abused and 'doesn't remember it' or had some early 'narcissistic injury'. Simply, she was born like it.

So, yes abuse often causes mental illness, but its not always the case by any means.