Sorry about replying to my own post. Just doing some thinking out loud.
I think it is conveying the idea "It's bad for a child to be an only child" (ie being around adults instead of children, having all the focus on that child with no siblings ot take some of the heat, etc), thus it would translate as "I was an only child about whom it was said "it's bad for him to be an only child." Not "sick," but "bad (ie unhealthy):" "It's unhealthy for a child to be an only child."
Could anyone confirm this idea?