He'd had heart problems and was on a list for a Heart Transplant but sadly died before it could happen.
Iron Butterfly were pioneers of "psychedelic rock" & may be considered by many to be a "one-hit-wonder" but that One Hit, "In-A-Gadda_Da_Vida", was one of the most iconic songs of the 60s!
Here are links to 3 of their other songs that feature Lee's distinctive bass style:
In The Time Of Our Lives
I met Lee briefly once after a concert in 1969. He was very friendly and cordial.