Manet died of untreated syphilis and rheumatism, which he contracted in his forties. The disease caused him considerable pain and partial paralysis from locomotor ataxia in the years prior to his death. He continued to work until his premature death in April 1883.
His left foot was amputated because of gangrene, an operation followed 11 days later by his death. He continued to work until his premature death at age 51 in Paris in April 1883.
Painter Paul Gauguin was also said to have suffered from syphilis.
This is the PDF of the NYTimes' original announcement of Manet's death from 1883: PDF.
Manet began his career late in life, but he developed his talent quickly. His last works revealed no deterioration in quality. What would he have gone on to produce in his later years will, sadly, remain unknown.
Édouard Manet. Wikipedia.
Notable syphilis-infected people in history. Wikipedia.
Manet's Death. MyStudios.