It’s worth remembering that Syphilis can be passed from mother to unborn child also. Sexual activity does not have to be involved, which can make it very difficult to spot. Babies can be born blind, or deaf, but have no external symptoms.
Not sure if this relates directly to Beethoven or Schubert, but if a man contracts Syphilis through sexual contact, he can pass it to his wife. If she then becomes pregnant, even if she does not show symptoms, it can be passed to the child. All good fun eh? Loads of it in our history, the bug can survive for a number of generations too.