Nazi system for imprisoning those consider "enemies of the state." Many different groups and individuals were imprisoned in concentration camps: religious opponents, resisters, homosexuals, Jehovah's Witnesses, Roma and Sinti (Gypsies), Poles, and Jews. Concentration camps were further subdivided into labor camps and death camps. Before the end of World War II, several thousand of these concentration camps were operating throughout Europe, in all countries conquered by the German army, especially Poland, Austria and Germany. (Also called "extermination camps.") These were concentration camps created for the sole purpose of killing people. Victims were murdered in assembly-line fashion oftentimes in gas chambers, and their bodies burned in open fields or crematoria, or buried in mass graves. The Nazis operated six death camps: Sobibor, Belzec, Treblinka, Chelmno, Auschwitz and Majdanek. Many concentration camps, such as Bergen-Belsen, Mathausen and Dachau, were also considered death camps because thousands were killed there by starvation, mistreatment, and disease. The Nazis revived the medieval 16th-century term "ghetto" to describe compulsory "Jewish quarters" in the poorest sections of the cities and towns they had conquered. Ghettos were closed off by walls, or fences made of wood and barbed wire. Entire families were imprisoned in ghettos, including young children and the elderly. Ghettos were extremely crowded and unsanitary. Lack of food, clothing, medicine, and other supplies, severe winter weather, and the absence of adequate municipal services led to repeated outbreaks of epidemics and to very high mortality rates. With the implementation of the Final Solution in late 1941, most ghettos were systematically destroyed. Residents were either shot in mass graves located nearby or deported, usually by train, to death camps. The largest ghetto in Poland was the Warsaw Ghetto (pop. 450,000); other major ghettos were established in the cities of Lodz, Krakow, Bialystok, Lvov, Lublin, Vilna, Kovno, Czestochowa, and Minsk.