I thought syphilis was curable.
Syphilis is a curable disease with prompt diagnosis and treatment. However, if treated too late, there may be permanent damage to the heart and brain even after the infection is destroyed. I remember in my Abnormal Psychology class studying a syphilitic brain. It was pretty nasty looking.
For the first phase of the infection, which can last for decades, it is curable. After it get to the brain though it is not curable since the blood brain barrier prevents antibiotics from working.