Seemingly mild mono (Epstein-Barr) absolutely ruined the brain of one of the most talented dual-major Mechanical/Electrical engineers at my university in his last year. Saw him crying in more than one of our professors offices after he tried to come back because he couldn’t think straight enough to do 8th grade math anymore. Never recovered to my knowledge of a few years after, but it’s been a few years since I’ve heard anything of him. Medical imaging, at that time, purportedly showed nothing wrong with his brain. No help to be had.