Any disease that is passed from one individual to another during sexual intercourse or other types of sexual activity. They include gonorrhoea, caused by the bacterium Neisseria gonorrhoeae; syphilis, due to infection by the bacterium Treponema pallidum; genital herpes, which is caused by a herpesvirus; chlamydia, caused by the bacterium Chlamydia trachomatis; trichomoniasis, caused by the parasitic protist Trichomonas vaginalis; and AIDS, resulting from infection with HIV, a retrovirus. Pubic lice and scabies mites are also transmitted by close sexual contact. The transmission of sexually transmitted diseases can be reduced by limiting the number of sexual partners and by the use of condoms (see birth control), which reduces the risk of contact with body fluids that harbour the microorganisms that cause these diseases.

https://www.nhs.uk/Conditions/Sexually-transmitted-infections/pages/introduction.aspx Extensive account of STDs from NHS Choices