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 killed millions of people.
An earlier term, "venereal disease" (named after Venus, the Roman goddess of love), began to fall out of favor after the rise in the 1980s of STDs such as AIDS, which at the time was incurable and often fatal. The cavalier attitude towards VD seen by many as "no big deal, just get a penicillin shot" meant a new term was needed so people would take STDs more seriously.
While STDs are obviously a serious problem, they have also been used to incite a number of moral panics over the years. They are often cited as a reason to clean up purported epidemics of prostitution or pornography (although sex workers are often more aware of STDs than the general public, since their job depends on avoiding or mitigating the risk of them; according to one source, professional porn actors "are 80 percent less likely to have an STD than members of the public of the same age"), and are the subject of social guidance films which encourage abstinence. The teaching of abstinence-based sex education (accompanied by a disdain for contraceptives) has been disastrous for limiting the spread of STDs, as teachers may very well convince some of their students not to use condoms, but can't stop horny students from having sex. As condoms substantially reduce the risk of spreading these diseases, the lack of their use promoted by abstinence programs (which also tend to include "false information about the effectiveness" of birth control) helps to increase both the number of STDs and unwanted pregnancies.
In the 1980s and 1990s, widespread homophobia was fuelled by the perception that the LGBTQ+ community was spreading disease through their rampant, irresponsible and highly sexualised culture. The community was often blamed for all STD-related deaths, even those which were spread by non-LGBTQ+ victims. The American government usually funded programs to help eradicate (or at least mitigate) these epidemics, but also showed a serious disinterest in helping out with what they saw as an irresponsible gay disease, with Ronald Reagan and Jerry Falwell (among others) literally describing AIDS as a punishment from God because "illicit sex is against the Ten Commandments." Some of the most charming of Christian evangelists laughed hysterically at the plethora of the rather public and painful deaths of gay men as their loving God's revenge; that is until some of the pastors and members of their families or people they knew started dying from the disease themselves. Evangelicals may discriminate, but HIV does not. The American government only started treating AIDS seriously once the number of non-LGBTQ+ deaths began rising rapidly. It is estimated that thousands of people could have avoided lifelong suffering or death if the government had acted as quickly as they did when faced with other epidemics.
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.
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 HSV-2)
HSV can appear on either the lips or the genitalia. It usually presents as a painful, bumpy rash, sometimes with fever. Traditionally, HSV-1 only appeared on the mouth and HSV-2 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. The virus is extremely common: "[u]p to 90% of people around the world have at least one form of HSV." A large portion of those infected with either type don't experience symptoms and thankfully herpes is not a particularly dangerous disease for most people, though immune deficiencies or pregnancy might change that. However, the virus is highly contagious, and remains so until the sores have completely healed. It should also be noted that HSV-1 can be spread "in non-sexual ways", and many people with oral herpes probably "got it when they were kids."
There is currently no vaccine or cure for either type of HSV.
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 (such as the 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. While there does exist a vaccine for Hepatitis B, one has yet to be developed for Hepatitis C.
Neisseria gonorrhoeae (gonorrhea; also known as "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 (syphilis)
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. 
The first recorded outbreak of syphilis in Europe occurred during the Renaissance, and soon "gained notoriety as the ‘new plague’ which was so intimately and erotically tied to the ‘act of Venus’ that it gave derivation to ‘venereal disease’."
Trichomonas vaginalis (also known as "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 (also known as "crabs")
This presents as itching in the groin, and often the small parasites (colloquially known as crabs) are easily visible.
In the Middle Ages, pubic lice were so common that women (particularly prostitutes) would shave off their pubic hair before covering their vagina with a wig called a merkin (no, that's not a joke). Some sources also claim that the wig was used to hide the presence of other more serious STDs, such as "syphilitic pustules and gonorrhoeal warts in the genital area", although "there doesn’t seem to exist much in the way of any real evidence to support this claim". In fact, the first appearance of the merkin in historical sources dates back to 1450, while syphilis was not recorded in Europe until 1495.
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.