Bugs in the belfry tries to answer whether Nietzsche was already demented when he wrote his last few works:
In 1888, a sickly Nietzsche wrote the tracts ‘Twilight of the Idols,’ ‘The Antichrist,’ ‘Ecce Homo,’ and ‘The Case of Wagner’ in a burst of productivity. But the following January in Turin, he flung his arms around the neck of a horse being flogged, collapsed in the piazza, and swiftly descended into a raving dementia brought on — as records of a young Nietzsche’s treatment for syphilis 30 years earlier would appear to indicate — by paresis. So was Nietzsche suffering, as many have argued, from incipient paresis when he wrote ‘Twilight of the Idols,’ et al? [...] Or did the philosopher go mad from some other cause all of a sudden, in the space of a single day, as others prefer to believe?
A microbiologist by the name of Margulis believes that the spirochete Treponema pallidum, the lively, corkscrew-shaped bacterium that causes syphilis, can lay dormant for years before reviving explosively:
But is it possible for paresis to appear overnight, instead of slowly? Margulis believes it is, and as evidence points to studies of microbial mat samples taken from Eel Pond in Woods Hole and kept in a jar in a UMass-Amherst lab. Although no typical spirochetes were found in these samples, Margulis recounts, when food and water known to support spirochete activity were added to some samples, spirochetes that could only have been been lying dormant suddenly awoke from their slumber. Extrapolating from these experiments, Margulis argues that inactive Treponema pallidum spirochetes had been hiding out in Nietzsche’s tissues ever since his syphilis treatment some 30 years earlier.”
But on January 3, 1889 in Turin,” Margulis concludes, channeling Vincent Price, “armies of revived spirochetes munched on his brain tissue. The consequence was the descent of Nietzsche the genius into Nietzsche the madman in less than one day.”