He had eye surgery that worsened his eyesight

In 1750, Bach experienced cataracts, which affected his vision. He sought the advice of Dr. John Taylor as the condition grew worse, and the doctor suggested having the issue surgically fixed. This physician was regarded as a quackery of the 18th century.

Using a blunt instrument, the cataract was pushed deep into the eye during the "couching" technique. Following surgery, Taylor gave the composer ocular drops made of pigeon blood, mercury, and sugar crushed. It was a failure.

Unfortunately, the quality of the operation and level of understanding of the problem were not at their finest at the time. The surgery didn't work out. Bach had full vision following the procedure for a very brief time before being completely blind. He experienced multiple illnesses after the surgeries, which contributed to his quick passing.

Around the same time, George Friedrich Handel underwent cataract surgery with the same surgeon. Handel became completely blind as a result of this surgery by 1752.