Yes, if one partner has an STD, and sleeps with the other partner, (unprotected) then guess what.....chances are extremely high that partner #2 gets infected. Pay a visit to your doctor if you are worried about this, STD's left untreated can become very serious.
Yes, it is possible for one partner to have an STD without passing it on to the other, however, if they have continued sexual contact it is extremely unlikely that the STD would not be passed on at some point, and sooner rather than later.
The only way to prevent the other party from contracting the STD is to use condoms. Other forms of contraception like the pill will protect against pregnancy, but not against STDs.
If you want more information you should visit your local sexual health clinic where they will be able to answer any questions, and also provide you with some free contraception.
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