Sexually transmitted diseases, or STDs, also referred to as sexually transmitted infections or STIs, do not refer to any one disease but include more than 25 infectious organisms that are transmitted through sexual activity and the dozens of clinical syndromes that they cause. Examples of the common STDs include:

Acquired Immune Deficiency Syndrome (AIDS)
Chlamydia
Genital Herpes
Genital Warts/HPV
Gonorrhea
Hepatitis A
Hepatitis B
Herpes, Herpes Simplex Virus (HSV)
Human Immunodeficiency Virus (HIV)
Human Papillomavirus (HPV)
Pubic Lice (crabs)
Retrovirus infections other than HIV (HTLV)
Scabies (mites)
Syphilis

STDs are almost always spread from person to person by sexual intercourse, most commonly by anal or vaginal intercourse but also through oral sex. Some STDs, such as hepatitis B or HIV infection, are also transmitted through blood-to-blood contact, through the sharing of needles or equipment to inject drugs, body pierce or tattoo. Pregnant women with STDs may pass their infections to infants during pregnancy or birth or through breast feeding.