What disease was Morrie diagnosed with?

In the book Tuesdays with Morrie, Mitch Albom tells of the time he spent with his dying former sociology professor, Morrie. While studying at Brandeis University, Albom took numerous sociology courses taught by Morrie, and at graduation, Albom promised to keep in touch with Morrie. However, life gets in the way and Albom fails to keep in touch.
After seeing an ailing Morrie on "Nightline" one night, Albom reconnects with his former professor and discovers he has been diagnosed with ALS, also known as Lou Gehrig's Disease, which affects the nervous system and muscles and eventually kills those who suffer from it. Morrie finds he must give up many of his favorite activities, including dancing, and he requires constant care from his wife, Charlotte.
Soon, Albom is visiting Morrie every Tuesday to hear his life lessons and help care for Morrie, physically and emotionally. However, Morrie's life lessons are much more impactful for Albom, teaching Albom both the sanctity and fragility of life. In the end, Morrie dies from ALS, and Albom publishes a book, Tuesdays with Morrie, recounting the many lessons he learned from his former professor.