Heather Corinna replies:
We call STIs -- sexually transmitted infections -- what we do because that is usually how they are transmitted, but not ONLY how they are transmitted. For instance, Hepatitis is spread via blood, so while a person could get it sexually, they could also get it via an infected needle during intravenous drug use or a tattoo. Genital herpes, gonorrhea or syphilis can be spread from mother to infant during childbirth, and HIV or cytomegalovirus can be transmitted that way or via breastfeeding.
A few STIs are more frequently spread by casual contact. Trichomoniasis or pubic lice, for instance, can be transmitted by sharing damp towels, clothing or linens. And most people contract HSV-I, oral herpes, by casual, non-sexual contact, usually in childhood: it can then later be transmitted sexually.
For the most part, you're going to know if you have any of those. People with oral herpes will usually have had cold sores. People with pubic lice are itching and scratching like nobody's business. If you get general checkups from a general doctor every year, chances are pretty good that if you have something through casual contact -- or contracted an infection at birth -- your doc would have caught it by now.
Of course, when it comes to you, it's a lot easier to assess that because you know if you have or have not had any sexual partners, for sure. With other partners, we do have to remember that not only is everyone not truthful with partners -- especially if they feel ashamed about something, or want a specific result, such as not having to use safer sex -- but some people aren't sure what sexual partnership even means. If you ask a heterosexual partner if they've had sex before, for instance, they're likely to answer based on if they have had vaginal intercourse before, especially if they don't realize that STIs can be spread through other sexual activities. Too, younger people can often be really invested in having their first time be with a partner who is also new to partnered sex, so the pressure to say you've not been with anyone else before can be big.
So, with any partner you may have in the future, what you want to do to play it safe is to practice safer sex for six months before going without latex barriers (like condoms for genital intercourse of any kind or male oral sex). That's six months of latex barriers for any kind of genital sex, six months of monogamy, and two full and negative sets of tests for sexually transmitted infections -- one at the start of partnership, one six months later. At that point, if everyone is negative and has been (and stays) monogamous for that time, your STI risks will be very low.
For more on what STIs are transmitted how, click here: STI Risk Assessment: The Cliff's Notes. You can also click through the links in those lists to find out more about a given infection.
For information on how to practice safer sex, click here: Safe, Sound & Sexy: A Safer Sex How-To.