How Does Syphilis Damage the Brain?

Andrew Scull says syphilis damages the brain in its tertiary stage, when the organism that causes syphilis resides in the brains of people suffering from GPI.

Suspicions had tied the disorder to sex and moral dissolution. In the early 20th century, at the Rockefeller Institute, the actual origins turned up. Psychiatry had been looking for the underlying pathology behind mental diseases, and GPI was a case where that cause was found in the brain. In the primary phase, syphilis brings pain, then goes underground. The person thinks it is gone. It lurks the way the chickenpox virus lurks and can surface years later, and it keeps doing insidious damage. Sometimes it attacks the heart, and people drop dead of a heart attack in their 40s. The death is called a natural heart attack, but the cause is the syphilis.

Finding the syphilis organism in the brain gave psychiatry a concrete pathology for a mental disease long blamed on sex and moral dissolution.