Does anyone know what he died of?

I'm watching him in an episode of Alice (the other guest star was Jay Leno), and I recognized his voice. I knew I'd seen him somewhere before. I came here to look through his credits, and thought maybe I'd seen him in Laverne and Shirley, but after scrolling all the way down, when I saw him associated with Welcome Back Kotter, that's where it hit me.

But I was very surprised to see he died a few years back. He wasn't that old, so I'm assuming some sort of disease, but does anybody know exactly what it was?

Thanks.