John XXIII died of peritonitis caused by a perforated stomach at 19:49 local time on 3 June 1963 at the age of 81, ending a historic pontificate of four years and seven months. He died just as a Mass for him finished in Saint Peter's Square below, celebrated by Luigi Traglia. After he died, his brow was ritually tapped to see if he was dead, and those with him in the room said prayers. Then the room was illuminated, thus informing the people of what had happened. He was buried on 6 June in the Vatican grottos. Two wreaths, placed on the two sides of his tomb, were donated by the prisoners of the Regina Coeli prison and the Mantova jail in Verona. On 22 June 1963, one day after his friend and successor Pope Paul VI was elected, the latter prayed at his tomb.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): What did John XXIII die of?
peritonitis