Sexually transmitted disease
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Sexually transmitted diseases (STDs, or STIs, "sexually transmitted infections") comprise a large and diverse set of disease entities transmitted through various sexual means and sometimes through misuse of medical equipment. They include viruses, bacteria, protozoa, and arthropods. Many of these diseases can cause great harm or become widely spread before the symptoms of the disease are even detected. Some diseases are relatively harmless, some are painful and debilitating and a few have meant the end of life for millions of people.
An earlier term, "venereal disease", began to fall out of favor after the rise of herpes and AIDS in the early 1980s, in part because of the association of VD with "no big deal, just get a penicillin shot".
STDs have been an excuse for moral panics over the years, often cited as a reason to clean up a purported epidemic of prostitution or porn shops, and the subject of social guidance films teaching abstinence which even oppose teaching the use of condoms which substantially reduce the risk of spreading these diseases. This usually religiously driven campaign to avoid STDs utterly fails because teaching horny teenagers to just not have sex instead of how to do it safely drives up pregnancies and STDs.
In the '80s and '90s, the AIDS epidemic (amongst a few other diseases) was the most popular method of homophobia, and the LGBT community was often blamed for the entirety of deaths. Some of the most charming of evangelists laughed hysterically at the plethora of the rather public and painful deaths of gay men as their loving God's revenge.
List of STDs and their signs and symptoms
This list is not complete, and many of these disease agents can be transmitted by non-sexual means. Because there are so many different infections, symptoms can vary greatly. Your doctor can give you a great deal of information and should be consulted early to prevent permanent health problems.
Human Immunodeficiency Virus (HIV)
In the early stages, HIV can cause fevers, rashes, joint pains, and other non-specific symptoms. These usually occur within weeks after exposure. If untreated, HIV infection usually leads to Acquired Immunodeficiency Syndrome (AIDS) within eight to ten years. The symptoms of AIDS can be anything, but common presentations include weight loss, night sweats, unusual skin lesions, and diarrhea.
Herpes Simplex Virus (HSV 1 and 2)
HSV can appear on either the lips or the genitalia. It usually presents as a painful, bumpy rash, sometimes with fever. Traditionally, HSV1 only appeared on the mouth and HSV2 only appeared on the genitalia, giving them the names "cold sores" and "genital herpes" respectively. However, what with all the oral sex those darned kids these days engage in, either infection can now appear in either place. A large portion of those infected with either type don't experience symptoms and Herpes is not exactly a dangerous disease for most people, though immune deficiency or pregnancy might change that.
Human Papilloma Virus (HPV)
HPV often presents as anogenital warts, or as an abnormality on a pap smear. These lesions can be pre-cancerous. There is a vaccine against HPV which is currently recommended for females before they become sexually active, however, vaccinating boys as well might help further reduce the incidence of HPV. While most people who have HPV have no major problems, HPV is one of a few viruses that can cause cancer - especially in the parts of the body it comes into contact with most (penis, anus, mouth, vagina and so on).
Hepatitis B and C Viruses (HBV, HCV)
These are often asymptomatic, but over time can lead to cirrhosis and liver cancer. If any symptoms are present, the illness can look like any hepatitis, that is, jaundice, change in urine or stool color, abdominal swelling, and fatigue.
Neisseria gonorrhoeae (gonorrhea, "the clap")
In males, this presents as a foul penile discharge. Women may have a vaginal discharge, or no symptoms at all.
Chlamydia trachomatis (chlamydia)
This presents similarly to gonorrhea, but is often asymptomatic, and can be present without the patient knowing they are ill. There are three different sub-types of the bacteria Chlamydia Trachomatis that cause Lymphogranuloma Venereum. Lymphogranuloma venereum is one of the maladies associated with gay bowel syndrome. Chlamydia can also be transmitted from an infected mother to her baby during vaginal delivery, causing conjunctivitis or pneumonia. 
Treponema pallidum (syphillis)
This organism causes syphilis, a multisystem disease. It responds well to antibiotics, but if left untreated, its late stages can cause dementia and death. It can also be transmitted from mother to baby across the placenta or during vaginal delivery, resulting in fetal or neonatal death in almost half of cases and possible disability for surviving infants. 
Trichomonas vaginalis ("trich")
In females, this usually presents as a foul vaginal discharge, but males are usually asymptomatic. Even though males are asymptomatic, partners of infected females must be treated.
Pubic lice ("crabs")
This presents as itching in the groin, and often small parasites are easily visible.
This often presents as itching, with rash being more prominent between the fingers and toes.
At the risk of being a massive killjoy, simply keeping your pants on and refraining from all similar such activities is the only 100% guaranteed way to keep a disease from being transmitted through sex. But quite often this is understandably the least desirable option.
Latex condoms provide protection against many STDs (including HIV), but not all (such as
public pubic lice); they are ineffective when not properly used or in the case of rupture.
Vaccines are not widely available for STDs. An HPV vaccine has recently been released for protection against certain, but not all, strains of HPV. A Hepatitis B vaccine also exists, but no vaccine yet exists for Hepatitis C.
For the idiots out there, people who have been sexually active before marriage can transmit their sexual diseases to their partner after marriage, therefore failure to use proper protection with a spouse can still lead to infection.
Periodic testing is recommended for those who indulge in promiscuity, so they will be aware (painfully aware) of any treatment they should obtain, past lovers they should make embarrassing calls to, and even more embarrassing disclosures they must make to future lovers (If you're very lucky, a clinic or therapist may do the calling for you to preserve anonymity). To maintain your privacy throughout the testing process, try using an online STD testing site.
Some STDs are curable; many are not. For further information, please consult a physician.