Capone had v.d., knew he had v.d., supposedly would not be treated for it because he didn't have any symptoms (common when the disease progresses form stage 1 into stage 2 into tertiary).
He started behaving erratically in prison, was admitted to the infirmary, where he was diagnosed with tertiary phase syphylis, form which there was no cure. At the request of the family, he was released early to seek medical treatment else where.