He keeps infecting me with STIs but denies it

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I have been married for eight years but the last one year has been very bad. I had some complications sometime ago and upon getting a checkup, the doctor was surprised to treat me for an STI. When I told my husband, he was totally pissed off and accused me of sleeping around, which really hurt me for I have never been unfaithful to him. We were both treated but the same problem has recurred twice thereafter, and the doctor said that one of us must be having a partner who was not treated, thus they keep re-infecting us. I keep wondering why he is taking me through all this. The doctor said if I keep getting re-infected, it could affect my ovaries but I am also worried about HIV. I really don't know what to do now but I am scared. He has refused to come clean with me yet all this is putting me in great danger. Please advise...{Susan G}