Bacterial
Chancroid (Haemophilus ducreyi)
Chlamydia (Chlamydia trachomatis)
Granuloma inguinale or (Klebsiella granulomatis)
Gonorrhea (Neisseria gonorrhoeae)
Syphilis (Treponema pallidum)
Fungal
Candidiasis (yeast infection)
Viral
Viral hepatitis (Hepatitis B virus)—saliva, venereal fluids.
(Note: Hepatitis A and Hepatitis E are transmitted via the fecal-oral route; Hepatitis C is rarely sexually transmittable, and the route of transmission of Hepatitis D (only if infected with B) is uncertain, but may include sexual transmission.)
Herpes simplex (Herpes simplex virus 1, 2) skin and mucosal, transmissible with or without visible blisters
HIV (Human Immunodeficiency Virus)—venereal fluids, semen, breast milk, blood
HPV (Human Papillomavirus)—skin and mucosal contact. 'High risk' types of HPV cause almost all cervical cancers, as well as some anal, penile, and vulvar cancer. Some other types of HPV cause genital warts.
Molluscum contagiosum (molluscum contagiosum virus MCV)—close contact
Parasites
Crab louse, colloquially known as "crabs" or "pubic lice" (Pthirus pubis)
Scabies (Sarcoptes scabiei)
Protozoal
richomoniasis (Trichomonas vaginalis)