Bruno was the youngest of six children born to Jewish parents in the city of Osijek in eastern Croatia when it was still part of the Austro-Hungarian Empire. He attended elementary school in Osijek and then completed his secondary schooling in Poega, a Croatian town where his parents had moved in 1923. In 1932 the Klein family moved to Zagreb, the capital of Croatia.
1933-39: While living with his parents in Zagreb, Bruno attended medical school until 1938, when he graduated. He specialized in internal medicine and continued his studies as an intern in one of Zagreb's hospitals. The outbreak of World War II in September 1939 had no direct effect on Bruno, because Yugoslavia remained a neutral country.
1940-44: In April 1941 Germany invaded Yugoslavia and Croatian fascists came to power in Zagreb. Because doctors were badly needed in nearby Bosnia, Bruno, unlike most of Croatia's Jews, was not deported to a Croatian concentration camp. Instead, he was sent to be a doctor in the town of Banja Luka. There he worked during the week, but every Saturday he had to present himself to the local police station, where he was locked up on weekends with common criminals, without food, water, or even a cot to sleep on.
Bruno was killed during an Allied bombing of Banja Luka on Monday, April 24, 1944. He was 29 years old.