Short answers, pulled from the story.
Mozart contracted smallpox in October 1767, during a family visit to Vienna. He showed the first symptoms on the 26th of October in Olmütz (today Olomouc, Czech Republic), though the disease's roughly 12-day incubation period indicates he was infected before the family fled Vienna.
Mozart was 11 years old when he contracted smallpox in 1767.
Mozart lost his sight for nine days during his smallpox illness, according to a letter written by his sister Nannerl in 1800. Ophthalmologist Richard H. C. Zegers later suggested this was not true blindness but rather the result of the pustular rash affecting Mozart's eyelids.
In a letter of the 22nd of February 1764 to his friend Lorenz Hagenauer, Leopold stated he left the matter to the grace of God rather than risk a procedure that could kill the child immediately. Inoculation at the time used live smallpox virus and carried a real risk of death, making it a genuinely uncertain choice rather than simple negligence.
Mozart was treated in Olmütz (today Olomouc, Czech Republic) in the home of Count Leopold Anton Podstatsky, dean of the cathedral and rector of the University there. Count Podstatsky's personal physician, Dr. Joseph Wolff, oversaw Mozart's care.
After recovering, Mozart wrote La finta semplice following a Vienna trip that extended to the 19th of January 1768. The opera went unperformed in Vienna due to intrigues among local musicians, and was eventually premiered in Salzburg after the family returned on the 5th of January 1769.