What killed Mozart at 35?

Nobody knows. The parish register recorded severe miliary fever, which was a description of a rash rather than a diagnosis. Modern candidates include acute rheumatic fever, a streptococcal infection leading to kidney failure, and trichinosis from undercooked pork - a paper argued for the last of these from a letter in which Mozart mentions eating pork cutlets 44 days before he died.

The poisoning story is fiction, and its origin is sad rather than sinister. Salieri, in confused old age in an asylum, appears to have said something to the effect that he had poisoned Mozart, and denied it in lucid moments. Pushkin turned it into a play, Rimsky-Korsakov into an opera, and Peter Shaffer into Amadeus - by which point a rumour from a dying man's delirium had become the thing everybody knows.

The pauper's grave is also a misunderstanding. Mozart was buried in a common grave outside the city, which was standard Viennese practice under Josephine burial reforms intended to be hygienic and egalitarian - elaborate funerals had been actively discouraged by law. He was not destitute at the end; he was earning well and spending faster. The exact location was not recorded because such graves were reused, which is why there is no body to examine.