Classification and terminology
The term "sexually transmissible infection" is favoured over "sexually transmitted infection", as the latter is more restrictive in consideration that there are other means of transmission. For instance, meningitis is transmissible by means of sexual contact, although it is not labelled as an STI. This discrepency is addressed by the probability of infection by means other than sexual contact. In general, an STI is an infection which 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 are not taken into account, e.g., blood transfusion, sharing of a hyperdermic needle). Thus, one may presume that if a person is infected with an STI (e.g. chlamydia, gonorrhea, genital herpes), that it was transmitted to them by sexual contact.

Humans are more susceptible to transmission of infection by means of sexual contact for two main reasons:

The penis and vulva contain mucous membranes. Mucous membranes differ from skin in that pathogens (viruses or bacteria) can easily gain entry into the human body.
Fluids containing pathogens are commonly released from the body during sexual contact, e.g., semen, blood, and vaginal lubricative secretion.
When both factors are considered, the probability of transmission of infection is far higher than other means of transmission (e.g., kissing, sharing cutlery, shaking hands).

They were commonly known as venereal diseases (VD); Veneris is in Latin the genitive (possessive) form of the name Venus, the Roman goddess of love) until some time around 1990, when public health officials introduced the new term in an effort to improve the clarity of their warnings to the public.

Although STD and STI are most often used interchangeably, some make distinctions between the two. As quoted from http://www.etharc.org, "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 their 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".

The term STD refers only to infections that are causing symptoms or problems. Because most of the time, people don’t know 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.

Depending on the STD, a person may or may not still be able to spread the infection if no signs of disease are present. For example, a person is much more likely to spread herpes infection when blisters are present (STD) than when they are absent (STI). However, a person can spread HIV infection (STI) at any time, even if they haven’t developed symptoms of AIDS (STD)."

All sexual behaviors that involve contact with another person or the bodily fluids of another person should be considered to contain some risk of transmission of sexually transmitted diseases. Most attention has focused on controlling HIV, which causes AIDS, but each STD presents a different situation.

As may be noted from the name, sexually transmitted diseases are transmitted from one person to another by certain sexual activities rather than being actually caused by those sexual activities. Bacteria, fungi, protozoa or viruses are still the causative agents. It is not possible to catch any sexually transmitted disease from a sexual activity with a person who is not carrying a disease; conversely a person who has an STD got it from contact (sexual or otherwise) with someone who had it, or their bodily fluids.

Although the likelihood of transmitting various diseases by various sexual activities varies a great deal, in general, all sexual activities between two (or more) people should be considered as being a two-way route for the transmission of STDs (i.e., "giving" or "receiving" are both risky).

Health care professionals suggest safer sex, such as the use of condoms in any sexual activity, but safer sex should by no means be considered an absolute safeguard. Abstinence from sexual activities involving other people will protect against sexual transmission of sexually-transmitted infections; however, it must be noted that STDs can also be transmitted through other activities involving transfer of and exposure to bodily fluids, such as blood transfusions and other blood products, sharing injection needles, needle-stick injuries (when medical staff are inadvertently jabbed or pricked with needles during medical procedures), sharing tattoo needles, and childbirth. These means of transmission make certain groups, such as doctors, haemophiliacs and drug users, particularly at risk.

Recent epidemiological studies have investigated the networks that are defined by sexual relationships between individuals, and discovered that the properties of sexual networks are crucial to the spread of sexually-transmitted diseases. In particular, assortative mixing between people with large numbers of sexual partners seems to be an important factor.

Since prostitutes tend to have large numbers of sexual partners, prostitution without the use of safer sex precautions has often been associated with the spread of sexually transmitted diseases. Some travellers such as truck drivers and sailors also often have high numbers of sexual partners (often prostitutes). However, sexually transmitted diseases are potentially transmitted in any form of sexual relationship, so it is important that all members of the community who are engaged in sexual relationships use safer sex precautions, regardless of the nature of their relationships.

It is possible to be an asymptomatic carrier of sexually transmitted diseases. In particular, sexually transmitted diseases in women often cause the serious condition of pelvic inflammatory disease.

Treatment

American poster propaganda targeted at World War II soldiers and sailors appealed to their patriotism in urging them to protect themselves. The text at the bottom of the poster reads, "You can't beat the Axis if you get VD." Images of women were used to catch the eye on many VD posters.
Spanish Civil War poster, produced by the Republican government, saying "Avoid VD - Just as dangerous as enemy bullets"Sexually-transmitted diseases have been well-known for hundreds of years — the English language has short words for two of the most common: the "pox" (syphilis) and "the clap" (gonorrhea).

Prior to the invention of modern medicines, sexually-transmitted diseases were generally incurable, and treatment was limited to treating the symptoms of the disease. The first venereal diseases clinic opened on January 31, 1747 at London Dock Hospital.

The first effective treatment for a sexually-transmitted disease was salvarsan, a treatment for syphilis. With the discovery of antibiotics, a large number of sexually-transmitted diseases became easily curable, and this, combined with effective public health campaigns against STIs, led to a public perception during the 1960s and 1970s that they had ceased to be a serious medical threat.

During this period, the importance of contact tracing in treating STIs was recognized. By tracing the sexual partners of infected individuals, testing them for infection, treating the infected and tracing their contacts in turn, STI clinics could be very effective at suppressing infections in the general population.

In the 1980s, first genital herpes and then AIDS emerged into the public consciousness as sexually transmitted diseases that could not be cured by modern medicine. AIDS in particular has an asymptomatic period which allowed the disease to be spread to others, followed by a symptomatic period which leads rapidly to death unless treated. Recognition that AIDS threatened a global pandemic led to public information campaigns and the development of treatments that allow AIDS to be managed by suppressing the HIV virus for as long as possible. Contact tracing continues to be an important measure, even when diseases are incurable, as it helps to contain infection.

There is now a recognition that safer sex is the most reliable way of decreasing the risk of contracting sexually-transmitted diseases, both curable and incurable.

Current STD Treatment (Cure)
Types and their causative organisms
Note: All of the diseases on this list can be transmitted sexually. Some of the diseases on this list are commonly transmitted in other ways besides sexually, for example, HIV/AIDS is also commonly transmitted through the sharing of infected needles by drug users, and candidiasis, whilst it can be sexually transmitted, is very often not associated with sexual activity.

Bacterial
Chancroid (Haemophilus ducreyi)
Chlamydia infection (Chlamydia trachomatis)
Donovanosis (Granuloma inguinale or Calymmatobacterium granulomatis)
Gonorrhea (Neisseria gonorrheae)
Lymphogranuloma venereum (LGV) (Chlamydia trachomatis serotypes L1, L2, L3. See Chlamydia)
Non-gonococcal urethritis (NGU) (Ureaplasma urealyticum or Mycoplasma hominis)
Syphilis (Treponema pallidum)
 

Viral
Cytomegalovirus
Hepatitis (Hepatitis A and Hepatitis E are transmitted via the fæcal-oral route, not sexually; Hepatitis C is probably not sexually transmittable)
Hepatitis B
Hepatitis D
Herpes /HSV (Herpes simplex virus)
Human Immunodeficiency Virus (HIV/AIDS)
Human papillomavirus (HPV)
Certain strains of HPV cause genital warts
Certain strains of HPV cause cervical dysplasias which can lead to cervical cancer