What is an STD (Sexually Transmitted Disease)?

A sexually transmitted disease (STD), also known as sexually transmitted infection (STI) or venereal disease (VD), is an illness that has a significant probability of transmission between humans or animals by means of human sexual behavior, including vaginal intercourse, oral sex, and anal sex. While in the past, these illnesses have mostly been referred to as STDs or VD, in recent years the term sexually transmitted infection (STI) has been preferred, as it has a broader range of meaning; a person may be ''infected'', and may potentially infect others, without showing signs of ''disease''. Some STIs can also be transmitted via the use of IV drug needles after its use by an infected person, as well as through childbirth or breastfeeding. Sexually transmitted infections have been well known for hundreds of years.

Until the 1990s, STDs were commonly known as ''venereal diseases'' : ''Veneris'' is the Latin genitive form of the name Venus, the Roman goddess of love. ''Social disease'' was another euphemism.

Public health officials originally introduced the term ''sexually transmitted infection'', which clinicians are increasingly using alongside the term ''sexually transmitted disease'' in order to distinguish it from the former. According to the Ethiopian Aids Resource Center FAQ, "Sometimes the terms STI and STD are used interchangeably. This can be confusing and not always accurate, so it helps first to understand the difference between infection and disease. Infection simply means that a germ—virus, bacteria, or parasite—that can cause disease or sickness is present inside a person’s body. An infected person does not necessarily have any symptoms or signs that the virus or bacteria is actually hurting his or her body; they do not necessarily feel sick. A disease means that the infection is actually causing the infected person to feel sick, or to notice something is wrong. For this reason, the term STI—which refers to infection with any germ that can cause an STD, even if the infected person has no symptoms—is a much broader term than STD." The distinction being made, however, is closer to that between a colonization and an infection, rather than between an infection and a disease.

Specifically, the term STD refers only to infections that are causing symptoms. Because most of the time people do not know that they are infected with an STD until they start showing symptoms of disease, most people use the term STD, even though the term STI is also appropriate in many cases.

Moreover, the term ''sexually transmissible disease'' is sometimes used since it is less restrictive in consideration of other factors or means of transmission. For instance, meningitis is transmissible by means of sexual contact but is not labeled as an STI because sexual contact is not the primary vector for the pathogens that cause meningitis. This discrepancy is addressed by the probability of infection by means ''other than sexual contact''. In general, an STI is an infection that has a negligible probability of transmission by means other than sexual contact, but has a realistic means of transmission by sexual contact (more sophisticated means—blood transfusion, sharing of hypodermic needles—are not taken into account). Thus, one may presume that, if a person is infected with an STI, e.g., chlamydia, gonorrhea, genital herpes, it was transmitted to him/her by means of sexual contact.

The diseases on this list are most commonly transmitted solely by sexual activity. Many infectious diseases, including the common cold, influenza, pneumonia, and most others that are transmitted person-to-person can also be transmitted during sexual contact, if one person is infected, due to the close contact involved. However, even though these diseases may be transmitted during sex, they are not considered STDs.

Bacterial

Chancroid (''Haemophilus ducreyi'')
Chlamydia (''Chlamydia trachomatis'')
Granuloma inguinale or (''Klebsiella granulomatis'')
Gonorrhea (''Neisseria gonorrhoeae'')
Syphilis (''Treponema pallidum'')

Fungal

Tinea cruris "Jock Itch" (Trichophyton rubrum and others)—Sexually transmissible
Candidiasis or "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 (liver cancer) 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/ AIDS (''Human Immunodeficiency Virus'')— venereal fluids
HPV (''Human Papilloma Virus'')— skin and mucosal contact. 'High risk' types of HPV are known to cause most types of cervical cancer, as well as anal, penile, and vulvar cancer, and genital warts.
Molluscum contagiosum (molluscum contagiosum virus MCV)—close contact

Parasites

Crab louse, colloquially known as "crabs" (''Phthirius pubis'')
Scabies (''Sarcoptes scabiei'')

Protozoal

Trichomoniasis (''Trichomonas vaginalis'')

Sexually transmitted enteric infections

Various bacterial (Shigella, Campylobacter, or Salmonella), viral (Hepatitis A, Adenoviruses), or parasitic (Giardia or amoeba) pathogens are transmitted by sexual practices that promote anal-oral contamination (fecal-oral). Sharing sex toys without washing or multiple partnered barebacking can promote anal-anal contamination. Although the bacterial pathogens may coexist with or cause proctitis, they usually produce symptoms (diarrhea, fever, bloating, nausea, and abdominal pain) suggesting disease more proximal in the GI tract.