(Reuters) -Billionaire Sam Zell, who built a real estate empire and was known for his bets on distressed assets, died at the age of 81 due to complications from a recent illness, a statement from his private investment firm said on Thursday. Born in 1941 to Polish parents who escaped to the United States during the German invasion of Poland, Zell took a deep interest in real estate very early on. He began his career managing student housing apartments as an undergraduate at the University of Michigan and founded his chief investment vehicle, Equity Group Investments, in 1968.