dr. ann mckee studied over 170 brains, including michael's. >> when you see this disease in a person that young, it's surprising, even shocking. >> reporter: cte which is identified only after death has been found in numerous nfl players like hall-of-famers frank gifford and junior seau, take a look at this video. a hit michael told cassandra was one of his hardest. he flies at an opponent, slamming into him. colliding with such force parts of his helmet break off. >> i think he had way more than ten concussions. he was seeing stars and then he had sensitivity to light. all the time. he had a lot of anger and it was >> reporter: they turned to doctors but cassandra says test results came up empty. >> i was so mad that nobody could help him. it was just upsetting. he felt so alone. >> reporter: she says michael did not die in vain. >> letting people know that this disease is out there. you don't know how many hits or how many concussions away you are from getting a disease like this, that's what is so scary. it could happen to anyone. >> reporter: for "good morning america," ryan smith, ab