Answer
Neurosyphilis, a severe infection of the brain caused by the bacterium Treponema pallidum, is the answer. It affected Gaetano Donizetti, a renowned Italian opera composer, leading to progressive mental decline and eventual death in 1848. Neurosyphilis can manifest years after the initial syphilitic infection, causing various neurological and psychiatric symptoms, including cognitive impairment, delusions, and mood disorders.