Sexually transmitted diseases (STD), also referred to as sexually transmitted infections (STI) and venereal diseases (VD), are illnesses that have a significant probability of transmission between humans by means of sexual behavior, including vaginal intercourse, anal sex and oral sex. Some STIs can also be contracted by using IV drug needles after their use by an infected person, as well as through any incident involving the contact of a wound with contaminated blood or through childbirth or breastfeeding.

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, and syphilis. There are several viral STIs including herpes simplex, HIV, and human papillomavirus (HPV). There are also parasite STIs, including the crab louse.Safer sex is a method of decreasing the risk of contracting sexually transmitted infections during sexual activity. 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 B, and some types of HPV. Causes and symptoms of std are different and treatment also varies according to the disease.​​