What do we mean by sexually transmitted diseases?
When we say "sexually transmitted diseases, STD" we refer to a group of infections caused by bacteria, viruses, and parasites, that are transmitted only through intercourse. There are more than 20 STDs and include:

1. STDs caused by bacteria as chlamydia, gonorrhea, syphilis, and chancroid. These diseases can be treated and cured with antibiotics.
2. STDs caused by viruses as human immunodeficiency virus) HIV/AIDS(, herpes simplex viruses type II (HSV-2), human papilloma virus (HPV), hepatitis B virus, and cytomegalovirus. These diseases cannot be cured, once they enter the body they will stay. However, they can be controlled.
3. STDs caused by parasites as Trichomonas, and other exoparasitic insects as pubic lice and scabies.

Among the general population, what gender and age groups are mostly affected?
Since these diseases are transmitted mainly through intercourse, both men and women can be affected. However, health consequences are usually more severe in women. This is because:

1. Some STDs in women can lead to pelvic infections which can result in scarring of the reproductive organs, which in turn can lead to infertility problems or ectopic pregnancies (pregnancy ouside the uterus).
2. STDs in women might be related to cancer of the cervix.
3. Some STDs can be transmitted from the mother to her baby during or immediately after delivery like, Chlamydia, gonorrhea, syphilis, herpes, hepatitis B, HIV/AIDS, and rarely with genital warts.

STDs can occur at any age and with any ethnic background, however, they are more common to occur at a young age among sexually active people.

Are STDs transmitted only through vaginal intercourse?
The route of transmission is not limited to vaginal intercourse, but includes anal, oral sex and skin to skin contact with sores like those which can happen with genital warts.  

What are the signs and symptoms of STDs?
Most of the time STDs are asymptomatic:

1. Chlamydia (kluh-mid-ee-uh): asymptomatic in 80% of women, 50% of men. It can manifest as discharge from the vagina or the penis, burning or pain during urination. If symptoms develop, they usually need at least 3weeks after exposure.  Mostly lead to chronic infertility problems.
2. Gonorrhea (gah-nuh-ree-uh): can manifest as discharge from vagina (yellow-green) or penis (yellowish-white), with or without painful urination. Can lead to pelvic inflammatory diseases and hence chronic infertility problems. Symptoms can occur 2-7days after exposure.
3. Syphilis (siff-ill-iss): usually starts with a painless genital ulcer (chancre) that goes away on its own within few weeks. Then, secondary stage might start with fever, rash, headache and achy joints. Finally, latent syphilis develops. Primary symptoms usually develop within 10days to 3months after exposure with an average of 3weeks. Syphilis can be very dangerous if left untreated since other major organs will be affected including the brain.
4. Trichomoniasis (trih-kuh-muh-nye-uh-sus): can manifest as grey or yellowish-green, foamy vaginal discharge that has a foul odor, with intense itchiness with or without painful urination. Men usually are asymptomatic, or with mild irritation only. Symptoms usually occur after 5-28 days after exposure. This infection is not dangerous to humans but can be very uncomfortable. In pregnant women, trichomoniasis can cause the baby to be born early or to be born with a low birth weight.
5. Genital herpes: usually manifests as genital blisters that heal within 10days, but as the virus cannot be eliminated it passes into periods of activation (where blisters reappear) and deactivation (where there are no symptoms). It can be transmitted from the mother to the baby during delivery, and from one person to another even in the absence of symptoms
6. Genital warts: caused by a group of viruses called human papilloma virus. More than 30 types of viruses are known to cause genital warts. It is asymptomatic most of the time, but when symptoms develop they take several weeks to several months after exposure. Warts appear as bumps or growths on the male or female genitalia, they can be flat or raised, single or many, small or large. They tend to be whitish or flesh colored. It is not always seen by the naked eyes, and sometimes affected person is not aware of them. When the warts disappear by treatment or by itself, the HPV virus is still there, though it may not spread as easily. It has been associated with development of cervical cancer in women.
7.  Chancroid (shang-kroid): common in tropical areas, cause ulcers that are filled with pus,
    usually very painful in men, women are often unaware of them. The most common symptoms in women are pain with urination and intercourse.. Symptoms
    occur between 4-10days of exposure. Chancroid has been associated with an increased risk of transmission of the human immunodeficiency virus (HIV), the cause of AIDS. In 50 percent of cases, the lymph node  glands in the groin become infected within five to eight days of appearance of initial sores.

 How can such diseases be diagnosed?

1. Gonorrhea: by swab and culture the site of infection.
2. Chlamydia: by vaginal swab and identification of the genetic material of the organism.
3. Syphilis: usually diagnosed by the following specific blood tests, Venereal Disease Research Laboratory (VDRL) test, and Rapid Plasminogen Reagent (RPR) test.
4. Chancroid: can be diagnosed by clinical examination and culture of the material from the ulcer
5. Herpes: Clinically by examining the blisters, and culture of fluid-filled blisters.
6. Human papilloma virus and genital warts by clinical examination, pap smear and biopsy.
7. Trichomoniasis can be diagnosed by clinical examination and microscopical exam of vaginal disharge women and urethral swab in men. If a patient has trichomoniasis, a doctor will typically also test for gonorrhea and chlamydia because these STDs sometimes occur together. 

How can STDs be treated?
Most of the STDs caused by bacteria can be treated and cured with antibiotics.

1. Gonorrhea: by penicillin, or others as ciprofloxacin (cipro®), azithromycin (Zithromax®).
2. Chlamydia: By Zithromax®, or doxycycline.
3. Syphilis: Benzathine penicillin, or doxycycline.
4. Chancroid: Zithromax® or cipro®

STDs caused by viruses cannot be cured but measures are taken to control the outbreaks or symptoms. Antivirals can be used for herpes, and 0.5% solution or gel of podofilox (podophyllotoxin), or local interferone injections can be used for genital warts. A vaccine has been introduced recently the target some strains of human papilloma viruses. Some countries are considering its inclusion among the regular vaccination schedule of the teens.

Exoparasitic infestation as with pubic lice can be treated with 1% cream rinse of permethrin or 1% shampoo of lindane. Scabies on the other hand need 5% cream of permethrin.

Trichomoniasis can be treated with metronidazole (flagyl®) antibiotic. Treatment of the sex partner might be indicated according to physician.

How can STDs be prevented?
The most effective way of prevention is total abstinence from sexual activity. However, if this is not possible, the following measures can help control the problem:

1. Have regular clinical checkups.
2. Be familiar with the common types of STDs and their manifestations.
3. Avoid anal sex.
4. Avoid intercourse during menstruation.
5. The use of male latex condoms is encouraged, but should be used correctly.
6. Washing the genitals, urinating, or douching after sex will not prevent any STD.
7. Refer to the physician upon initial manifestations to prevent disease progress.
8. Be aware that most STDs are not transmitted through toilet seats and door knobs, except for trichomonas. This parasite can live for about an hour on damp towels, washcloths, and bathing suits. If someone uses these towels or washcloths or puts on the bathing suit, the disease may be passed on that way. On the other side, most of these diseases can be transmitted from the mother to the baby during delivery.

References:

http://www.nlm.nih.gov/medlineplus/sexuallytransmitteddiseases.html
http://www3.niaid.nih.gov/healthscience/healthtopics/sti/default.htm
http://www.nichd.nih.gov/health/topics/sexually_transmitted_diseases.cfm
http://familydoctor.org/online/famdocen/home/children/teens/sexuality/275.printerview.html
http://www.emedicinehealth.com/sexually_transmitted_diseases/page9_em.htm
http://www.kidshealth.org/teen/sexual_health/stds/std.html
http://www.idph.state.il.us/public/hb/hbchancroid.htm