Leipzig, 1750

“The operation, disregarding that it had to be repeated, passed off badly. Not only was he unable to see: but his otherwise healthy body was cast over the pile with additional damaging medicines, and other treatments: so that for half a year he was sickly almost all the time…”Carl Philipp Emanuel Bach‘s evaluation of his father condition after the interventions performed by John Taylor.
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Bach’s life was very much free of disease. There is no record of any serious illness. With his first wife Maria Barbara, he had 7 children and 13 with Anna Magdalena, his second wife. Following his footsteps, 4 of his children became well known composers.
He was nearsighted from birth, which didn’t bother him very much until the age of 64. How close he was to total blindness at this point is not known. His creative ability however was retained, it has been said that during this last period of his life he had to dictate his compositions.
The reason for his blindness was senile cataract, which developed at the expected age. John Taylor* twice performed unsuccessful operations. Four months later he suffered a stroke, likely due to the obstruction of a brain vessel or a massive brain hemorrhage. Bach went into a coma and died. A direct causal relationship to Taylor’s operation is less likely than an indirect one; due to manipulation in both eyes an infection developed and on top of it application of damaging medicines weakened the patient. These affected negatively a generally asymptomatic preexisting condition.

http://www.ahsl.arizona.edu/about/ahslexhibits/diseasesofcomposers/bach.cfm