STD is the common abbreviation for a Sexually Transmitted Disease. It includes most diseases that are primarily spread through sexual intercourse or other sexual contact, because they inhabit only the mucous membranes of the reproductive tract. This includes Syphilis, gonorrhea, herpes, chlamydia, and HIV.
STDs also often share the ability to hide from the immune system in the parts of the reproductive system, where the immune system cannot reach. Many of them also avoid the immune system by being able to go through long periods of latency.