You can sue him IF he know that he had an STD, but despite knowing of it, failed to either warn you or take reasonable steps (e.g. condom) to prevent its transmission. In that case--when he knew--his failure to warm or take protective steps is at least neligent (careless); it may even be a deliberate wrongful act (i.e. if he wanted to infect you). That fault on his part could make him liable. But if he did not know he had the STD, he had no reason to warn you or take any particular precautions--that is, he did not nothing wrong. Without fault, there is no liability.
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