How did he get the disease ALS ?

Luckiest Man, by Jonathan Eig, is about the famous baseball player Lou Gehrig, his life, and his struggle with Amyotrophic Lateral Sclerosis. ALS is a progressive neurodegenerative disease, meaning that the nerves of the body weaken and break down progressively over time. ALS is not a disease that one "gets" in the same way that one catches a cold or the flu. Rather, the condition is the result of genetic and environmental factors. Some people inherit a genetic mutation which causes the onset of ALS, but the "sporadic" form is much more common. Lou Gehrig was most likely one of the people who developed the sporadic form, but if anyone in his family had passed on the ALS gene to him, it was not diagnosed or reported. In more recent years, it has been suggested that Gehrig may have developed ALS out of an underlying genetic predisposition which was exacerbated by brain trauma associated with his career in baseball.
http://www.alsa.org/about-als/what-is-als.html

https://www.simonandschuster.com/books/Luckiest-Man/Jonathan-Eig/9780743268936