I had it at 16, and like you I was down for a month. Barely could do anything - I don't recall bad mental effects but physically I was zero energy for a long time. I feel like my constitution never really recovered. Even today I get bouts of illness unlike anyone else my age or in my family, and I'm otherwise a very healthy person (healthy weight, work out regularly, etc.).

I never really thought about it until the long COVID discussions and learning more about things like this, but it really is something people barely understand today.