This fascinating article suggests that multiple concussions—which Lou Gehrig was known to have suffered playing both football and baseball—can lead to exactly the same types of neurological disease that doctors normally associate with Lou Gehrig's Disease (amyotrophic lateral sclerosis).
The Yankee legend had a well-documented history of significant concussions on the baseball field, and perhaps others sustained as a battering-ram football halfback in high school and at Columbia University. Given that, it’s possible that Gehrig’s renowned commitment to playing through injuries like concussions, which resulted in his legendary streak of playing in 2,130 consecutive games over 14 years, could have led to his condition.
“Here he is, the face of his disease, and he may have had a different disease as a result of his athletic experience,” said Dr. Ann McKee, the director of the neuropathology laboratory for the New England Veterans Administration Medical Centers, and the lead neuropathologist on the study.
In other words, it's possible that Lou Gehrig didn't actually die of Lou Gehrig's Disease.
Which is, at some level, impossible.