In general, sexually transmitted infections are just that—sexually transmitted! If you are not engaging in sexual activity, you are not at risk for STIs.
However, there are a few infections considered STIs that can spread non-sexually. For instance, pubic lice (also known as Crabs) can be spread through wet towels, loofahs, bedsheets, and wet clothing. Pubic lice can live outside the body for up to 24 hours in locations that are warm, dark, and moist, which makes it easier for them to spread without sexual activity. Additionally, according to the Center for Disease Control, 4% of all cases of Trichomononiasis are transmitted through hot tubs and wet towels.
Bloodborne STIs—like HIV, Syphilis, and Hepatitis B & C—can also be spread non-sexually. This most often occurs during IV drug use, medical needle sticks, or blood transfusions that did not receive proper screening. Again, it’s important to realize these infections would not be considered STIs in this scenario, since they were not spread sexually. (However, if you received them non-sexually, you could still transmit them to others through sexual activity.)
Last, it’s important to be specific about what we mean by “sex.” If you are engaging in genital to genital contact—even if you are not engaging in penetrative oral, anal, or vaginal sex—you are at risk for some STIs. In particular, HPV, Herpes, Syphilis (when a sore is present), and pubic lice can all spread through skin to skin contact.
If you are experiencing symptoms of an STI but haven’t been sexually active, you may want to read a bit about yeast infections and bacterial vaginitis. These are organic infections—they can occur without being transmitted from anyone—but they can be just as uncomfortable as some STIs. If you are experiencing pain, bleeding, itching, or discharge, make an appointment to see a medical provider soon.
Thanks for asking this (slightly complicated) question!