Arthritis, yes. The dr told us it isn’t hereditary, but a person is more likely to get it if it is in the family. I would call that hereditary!
That is one reason that I am not convinced about the head injury angle.
The brain is so complicated I wouldn’t rule out anything. What if playing football for 10 years BEFORE going pro causes minute damage that is repeated.
Rheumatoid arthritis is an autoimmune system gone haywire. Also want to ask another question...Did those who developed arthritis and Alzheimer’s suffer under stress for any length of time? Stress triggers the immune system. Mine kicked in during my son’s deployment.