The risk of contracting a sexually transmitted infection (STI) from unprotected sex depends on a few factors. If the sexual partner has had multiple sexual partners and or is high-risk partner eg. commercial sex worker, the risk of contracting an STI will be significantly higher. If you have multiple partners and engage in unprotected sex, the risk will be very much higher too.
If you or your partner is having an active infection or having blisters, sores and ulcers at the genital area, the risk of contracting an STI is also increased.
The best way to minimize the risk of contracting an STI is to engage in protected sex with a condom and limit the number of sexual partners. It is important to know your sexual partner's past sexual history and exposure too as the greater the number of partners, the higher the risk.