Bergen-Belsen (near Hanover, Germany) began as Stalag XI-C, a German POW camp designed to hold French and Belgian troops taken in 1940. Then, for a while it was used as a camp for Soviet prisoners, most of whom perished there.
In 1942 the camp was transferred to the SS for use as a 'Medical Recovery Camp'. In 1944-45 it was used as a temporary transfer point for slave laborers from the large camps in Poland who were being moved to avoid the advancing Soviet forces.
It eventually held as many as 60,000 prisoners and while it wasn't an extermination camp, it was hopelessly overcrowded and insanitary. Early in 1945 typhus swept through the camp and thousands died there every month, with more then 50,000 total deaths.
It was liberated on 15 April 1945 by British forces. They found 'walking skeletons', and the camp was littered with corpses. It was a scene of indescribable Horror. The camp was filmed by the British and the film was shown in British cinemas and worldwide. For most people it came as a shock.
When it was liberated by the British Army on 15 April 1945, they were greeted by a sight so horrific that initially the BBC (British Broadcasting Corporation) was unwilling to report it until 19 April 1945.
The British Army ordered German doctors and nurses to tend the sick and issued an immediate appeal for British personnel to volunteer to help there.