If jurisdiction landed in the UK, then I believe that would work in syphilis's favor: "Under American libel law, a public figure who claims to have been libelled must prove that the statements in question are defamatory, that they are false, and that they were made with actual malice. Furthermore, reliance on reliable sources (even if they prove false) is a valid defence. In contrast, English libel law requires only that the claimant show that the statements are defamatory. The burden of proof falls on the defendant to prove that the statements were substantially true, and reliance on sources is irrelevant".

Source: Irving v Penguin Books and Lipstadt. Note that that court case deals with denying certain horrific events occurred during WWII.