The composer Wolfgang Amadeus Mozart died on 5 December 1791 at the age of 35. The circumstances of his death have attracted much research and speculation. Some principal sources of contention are as follows.
Mozart scholarship long followed the accounts of early biographers, which proceeded in large part from the recorded memories of his widow Constanze and her sister Sophie Weber as they were recorded in the biographies by Franz Niemetschek and Georg Nikolaus von Nissen. For instance, the important biography by Hermann Abert (1923/2008:1305-9) largely follows this account. The following is a summary of this view.
When in August 1791 Mozart arrived in Prague to supervise the performance of his new opera La clemenza di Tito (K. 621), he was "already very ill" (Abert, p. 1305). During this visit, Niemetschek wrote, "he was pale and expression was sad, although his good humour was often shown in merry jest with his friends." Following his return to Vienna (mid September 1791), Mozart's condition gradually worsened. For a while, he was still able to work and completed his Clarinet Concerto (K. 622), worked toward the completion of his Requiem (K. 626), and conducted the premiere performance of The Magic Flute (K. 620) on 30 September. Still, he became increasingly alarmed and despondent about his health. An anecdote from Constanze is related by Niemetschek:
On his return to Vienna, his indisposition increased visibly and made him gloomily depressed. His wife was truly distressed over this. One day when she was driving in the Prater with him, to give him a little distraction and amusement, and they were sitting by themselves, Mozart began to speak of death, and declared that he was writing the Requiem for himself. Tears came to the eyes of the sensitive man: 'I feel definitely,' he continued, 'that I will not last much longer; I am sure I have been poisoned. I cannot rid myself of this idea.'