In most instances, STDs are passed from an infected person to another person during sexual activities, through contact with the mucous membranes of the penis, vagina, mouth and rectum. Such activity includes vaginal, oral and anal intercourse. Gonorrhea and chlamydia also can be transmitted by fingers to eyes.
A sexually transmissible infection (STI) is any infection or disease that can be passed from one person to another during sexual activity. Sexually transmissible infections include chlamydia, herpes, gonorrhoea, syphilis, genital herpes, scabies, pubic lice (crabs), hepatitis and HIV (the virus that causes AIDS).