Can I sue someone for neglect to disclose the fact that they had an STD?

At a minimum, you'd have to show that she knew *at the time* she had relations with your boyfriend that she had a disease but knowing of it, failed to tell him, since obviously, if she herself did not know, she did nothing wrong. 
Also, while the law is fairly clear that he could sue her if she knew but failed to tell him, *you* may not be able to. To oversimplify, the law limits how far "out" a person can be from an act or event and sue, because a person is only liable for the reasonably foreseeable consequences of the action. Once many other decisions or factors come into play between the act and the consequence, it is not reasonably foreseeable. She had sex with your boyfriend; she did not have sex with you and did not know you would be exposed indirectly to any disease she had. She did not cause your boyfriend, with whom you thought you had an exclusive relationship, to cheat on you. She did not cause your boyfriend to not take precautions, since he had been with another woman, to avoid infecting you (since disease was a possibility), or to (if he failed to tell you) not disclose promptly to you his cheating, so could make an informed decision whether to be intimate with him or to withhold intimacy until he was tested for a disease (or if he did promptly tell you, she did not cause you to have sex with him without insisting he be tested first); etc. That is, there are too many other variables, factors, and actions/decisions by other people in between her infecting him and you getting infected, so that a law might find a lack of "proximate cause," or an insufficiently foreseeable (by her) causal link, and so not hold her accountable to you, even if she had knowledge.