Wolfgang Amadeus Mozart was only eleven years old when he and his family arrived in Vienna from Salzburg. They were almost immediately caught in a smallpox outbreak: both he and his sister Nannerl were infected, and young Amadeus lost his sight for nine days as a result. Fortunately, both eventually recovered.
“Everywhere one could hear nothing but talk of smallpox, from which nine out of ten patients died,” recalledLeopold Mozart, Mozart’s father, in a letter about those difficult times.
One of Empress Maria Theresa’s daughters, Archduchess Josepha, also fell victim to the epidemic. The tragedy deeply affected the young Mozart, inspiring a rarely heard and profoundly moving canon entitled Ach, was müssen wir erfahren? (“Ah, what more must we endure?”).

Source: Wolfgang Amadé Mozart, Selected Letters and Documents. Rózsavölgyi és Társa, 2017