STD’s are caused by bacteria, fungi, viruses, protozoa, and parasites, most of which can pass through the mucous membranes of the penis, vulva, rectum, and urinary tract, as well as the mouth, throat, respiratory tract and eyes. Some pathogens (e.g. T. pallidum and Hepatitis B and C viruses) may additionally invade through skin breaks or abrasions even if minor. Others such as Herpes simplex virus and Human papilloma virus can also be transmitted by direct skin contact.

The main sources of infection are body fluids and excreta. All sexual behaviours that involve contact with the body fluids of another person should be considered a risk of transmission of STD’s. The transfer of and exposure to body fluids such as blood, whether directly by blood transfusion or through childbirth, or inadvertently by sharing injection needles or tattoo needles, or by sustaining needle-stick injuries during medical procedures are other avenues of transmission.

Most attention has focused on controlling HIV, but each STD presents a different situation. The dose of organisms required to cause infection varies with each pathogen, e.g. with transmission of infection being 50 – 100 times more likely for hepatitis B than for HIV. Depending on the STD, an asymptomatic person may still be able to spread the infection (e.g. HIV) but usually at a reduced rate. For example, a herpes infection is much more contagious when blisters are present.

In spite of a differing likelihood of transmitting STD’s by various sexual activities, they should all be regarded a two-way route for the transmission, i.e., "giving" or "receiving" are both risky although receiving carries a higher risk. STD’s that are easily transmitted from the mouth to the genitals or v.v. are much harder to transmit from one mouth to another.

Safer sex, such as the use of condoms, is the most reliable way of decreasing the risk of contracting STD’s during sexual activity, but is no absolute safeguard.

The two most frequent manifestations of STD’s are discharge and ulceration, and the presence of these is responsible for transmission and suggests specific disorders.