in these states where it's either criminally or civilly actionable to knowingly infect someone with an STD, how could you prove that the defendant isn't the party that contracted the disease rather than gave it?
It would be expensive BUT. Iirc there should be a way to test for the viral strain , so and do strains could be identified as related to the strain affecting the victim. After that it should be a game of finding the corcumstanvial evidence.