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A prisoner exchange or prisoner swap is a deal between opposing sides in a conflict to release prisoners. These may be prisoners of war, spies, hostages, etc. Sometimes dead bodies are involved in an exchange.
Geneva Conventions 
Under the Geneva Conventions, any prisoner, who due to illness or disability cannot contribute to the war effort is entitled to be repatriated to their home country. This applies regardless of number of prisoners so affected; the detaining power cannot refuse a genuine request.
Under the Geneva Convention (1929), this is covered by articles 68 to 74, and the Annex. One of the largest exchange programmes was run by the International Red Cross during the Second World War under these terms. Under the Third Geneva Convention of 1949, this is covered by articles 109-117.