Genital herpes is common. It is usually caused by HSV-2, but it can also be caused by HSV-1. About 12% of people in the US have been infected with HSV-2. Its possible you've been infected with HSV-2 in the past and don't know it, because not everyone who gets infected with HSV-2 develops symptoms (for example, ulcers or an ""outbreak""). You could ask your primary care provider to test you for antibodies to HSV-2, this would tell you whether or not you've already been infected.
If you've already been infected with HSV-2 in the past, then your partner can’t transmit to it you. If you have not been infected with HSV-2, then there are a few things you can do to protect yourself. Condoms reduce the chances of HSV transmission by about 50%. Another way to prevent HSV-2 transmission is for your partner to take a daily anti-HSV medication (like acyclovir or valacyclovir). This will reduce the chance that your partner will pass HSV-2 on to you. For more info about herpes, look here and to http://www.ashasexualhealth.org/stdsstis/herpes/
One study of people with genital HSV-2 who took a daily antiviral medication showed that people taking medication were both less likely to shed virus, and less likely to transmit the virus to their partners. In addition, people who both took daily medication and used condoms for intercourse had further reduced risk of herpes transmission in this study.