Not all STIs are symptomatic, and symptoms may not appear immediately after infection. In some instances a disease can be carried with no symptoms, which leaves a greater risk of passing the disease on to others. Bacterial STIs include Chlamydia, Gonorrhea, colloquially known as "the clap", and Syphilis. A fungal STI is Candidiasis (yeast infection). There are several viral STIs, including Viral hepatitis, Herpes simplex, HIV (Human Immunodeficiency Virus), and human papillomavirus (HPV). There are also parasite STIs, including the Crab louse, colloquially known as "crabs" or "pubic lice" and Scabies.

Healthcare professionals suggest safer sex, such as the use of condoms, as the most reliable way of decreasing the risk of contracting sexually transmitted diseases during sexual activity, but safer sex should by no means be considered an absolute safeguard. Prevention is key in addressing incurable STIs, such as HIV and herpes. The most effective way to prevent sexual transmission of STIs is to avoid contact of body parts or fluids which can lead to transfer with an infected partner. Not all sexual activities involve contact: cybersex, phonesex or masturbation from a distance are methods of avoiding contact. Proper use of condoms reduces contact and risk. Vaccines are available that protect against some viral STIs, such as Hepatitis A, Hepatitis B, and some types of HPV.