Concentration camps are places to torture or to force hard labor on groups of undesired people. Jews, homosexuals, Gypsies, and more groups were sent to Nazi concentration camps. Some people were executed because of their inability to work. Jews were also sent to killing centers to be murdered. Many people died in the Nazi concentration camps to disease, horrible treatment, overwork, and starvation between 1933, when the first concentration camp was created, and 1945. Most of the Nazi concentration camps were in Germany and Poland. All of those victims were members of what the Nazis thought were undesired people.
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http://www.ushmm.org/wlc/en/article.php?ModuleId=10005263