Sexually transmitted diseases (STDs) are caused by three types of organisms. These organisms include:

• Parasites. These are very small organisms that get nourishment from the person they infect (the host). Examples of sexually transmitted parasites include:
  
  
• Pubic lice. Also known as “crabs” or Phthirus pubis, pubic lice are parasitic insects usually found in the genital area on pubic hair. However, they may occasionally be found on other areas of coarse body hair, including the legs, armpits, mustache, beard, eyebrows or eyelashes. Pubic lice are most often spread through sexual contact, including vaginal intercourse and anal intercourse.
  
  Rarely, the parasites may also spread through contact with an infected person’s bed linens, towels or clothes. Pubic lice are most often found in adolescents, and are characterized by moderate to severe itching in the area covered by pubic hair. According to the National Institutes of Health (NIH), approximately 3 million new cases occur in the United States each year.
  
  
• Trichomoniasis. Trichomoniasis is caused by Trichomonas vaginalis, a single-celled parasite that most commonly affects the vagina in women and the urethra in men. Transmission may occur during penis-to-vagina or vulva-to-vulva contact. It may also be spread through contact with damp, moist objects such as towels or wet clothing. Trichomoniasis is one of the most common curable STDs in young women. Some experts believe that trichomoniasis may also increase the risk of human immunodeficiency virus (HIV) transmission.
  
  
• Scabies. Scabies is caused by infestation with Sarcoptes scabei, a microscopic mite. It may be transmitted through skin-to-skin contact, including vaginal and anal intercourse, or it may result from nonsexual contact such as sharing clothes, towels or bedding. Scabies is characterized by small bumps or rashes on the skin and intense itching. Possible complications include secondary infections caused by skin damage from frequent scratching.
  
  
• Viruses. These complex molecules invade the cells of their host, where they are capable of growing and multiplying. Viral STDs are commonly transmitted during sexual activity, including vaginal intercourse, anal intercourse and oral sex (mouth-to-genital contact).
  

  

• Human immunodeficiency virus (HIV)/Acquired immunodeficiency syndrome (AIDS). HIV, the virus that causes AIDS, is carried in blood, semen, vaginal fluids and breast milk. In addition to being transmitted sexually, HIV may also be spread by sharing needles used to inject intravenous drugs or from mother to child (during delivery or breastfeeding). According to the U.S. Centers for Disease Control and Prevention (CDC), approximately 900,000 people in the United States are currently infected with HIV, which impairs the body’s ability to fight infection. As a result, people with AIDS are highly susceptible to many life-threatening diseases and certain forms of cancer. In the early 1980s, HIV was also transmitted via blood transfusions and blood products. However, all donated blood products, in addition to organs for transplant, are now screened for HIV in the United States and other developed countries.
  
  
• Genital herpes. This STD is caused by the herpes simplex virus (HSV). The two types of HSV include type 1 (HSV1) and type 2 (HSV2). Both types can cause genital herpes but most cases are caused by HSV2. The viruses are usually spread through contact with sores during sexual activity. They may also be spread through contact with skin that does not appear to be broken or have a sore, or from an infected woman to her infant during delivery.
  
  The herpes virus also causes the typical cold sore. Cold sores are usually caused by HSV1, but may also be caused by HSV2. Performing oral sex while having an active cold sore can cause transmission of the virus to the partner’s genital area. Rarely, genital herpes can also be transmitted through contact with a toilet seat or hot tub.
  
  Many people do not experience any signs or symptoms of genital herpes, but the condition may be characterized by the appearance of blisters on or around the genitals. According to the CDC, genital herpes affects about 45 million Americans. Genital herpes increases the risk of HIV transmission.
  
  
• Viral hepatitis. There are numerous types of hepatitis viruses, including the hepatitis A, B, C, D and E virus. Although the hepatitis B, C and D viruses can all be transmitted through sexual activity, the B virus is most easily transmitted in this manner. The CDC estimates that between 120,000 and 200,000 new cases of hepatitis B occur each year. Of these cases, as many as half are transmitted through sexual contact. Infection with the hepatitis B virus can result in severe complications including chronic hepatitis, cirrhosis and liver cancer.
  
  
• Human papillomavirus (HPV). HPV is caused by a group of more than 100 types of viruses, including the viruses that cause common skin warts. Of these various strains, more than 30 are sexually transmitted and live only in genital tissue. HPV may also be spread through skin-to-skin contact involving the genital area and, in rare cases, from mother to infant during delivery. HPV usually causes no symptoms. However, some patients may develop small, flesh-colored bumps known as genital warts. Several types of HPV are associated with the development of cervical cancer in women. This cancer is common in women outside the United States, but its U.S. incidence has dropped because of regular monitoring through Pap smears. HPV can infect the vagina, penis, anus, rectum, mouth and throat. According to the CDC, approximately 20 million people are infected with HPV in the United States. A vaccine for the strains of HPV that are most likely to cause cervical cancer has been approved by the FDA for use in female adolescents and young adult females.
  
  
• Bacteria. These single-celled organisms cling together in colonies to feed from the body of their host. They are commonly transmitted through sexual activity, including vaginal intercourse, anal intercourse and oral sex. Many sexually transmitted bacterial infections also increase the risk of HIV transmission. Examples of sexually transmitted bacterial infections include:
  
  
• Chlamydia. This STD is caused by the bacterium Chlamydia trachomatis. In addition to being transmitted sexually, chlamydia can also be passed from an infected woman to her baby during vaginal childbirth. In women, the bacteria typically infects the cervix and the urethra and may spread to the fallopian tubes and rectum. In men, the disease affects the penis and testicles. Following anal or oral sex, the rectum or throat may become infected. Sexually active teenage girls and young women are particularly susceptible to infection because their cervixes are not fully mature, allowing bacteria to penetrate more easily. Left untreated, chlamydia may cause pelvic inflammatory disease (PID), a major cause of infertility and ectopic pregnancy. Complications among men are rare.
  
  
• Gonorrhea. Gonorrhea is caused by the bacterium Neisseria gonorrhea, which can easily grow and multiply in warm, moist areas of the body. Such areas may include the cervix and uterus in women, the urethra in men, and the mouth, throat, eyes and anus in both men and women. In some cases, gonorrhea may spread further to the blood or joints. In addition to being transmitted through sexual contact, gonorrhea can be passed from a pregnant woman to her infant during delivery. Gonorrhea is characterized by thick discharge from the vagina or penis. The most common and serious complications of the infection occur in women and include PID, ectopic pregnancy and infertility. In men, the disease can lead to epididymitis, a condition of the testicles that can result in infertility.
  
  
• Syphilis. This STD is caused by the bacterium Treponema pallidum, which is passed through contact with syphilis sores. Sores mainly appear on the external genitals, vagina and anus or in the rectum. They may also appear on the lips, in the mouth or on the hands. Syphilis may also be passed from a woman to her child during delivery. Left untreated, syphilis may progress to a rash, and eventually affect the heart and central nervous system (brain and spinal cord). The CDC estimates that more than 60 percent of all reported primary and secondary syphilis (early stages of the disease that indicate recent infection) cases occur among men who have sex with other men. Syphilis rates have declined in the past 50 years and now occur mostly in urban areas. They sometimes spike up in cycles and then decline again.
  
  
• Chancroid. Chancroid is caused by the bacterium Haemophilus ducreyi. This STD is primarily found in developing countries in Africa and Asia. It is generally rare in the United States, with only a few hundred cases diagnosed each year. However, chancroid occurs in periodic outbreaks in the United States, the last of which was in the 1980s. Chancroid is characterized by the development of one or more small bumps in the genital area that become ulcers within one day of their appearance. The lesions may resemble genital herpes or syphilis ulcers.
  
  
• Granuloma inguinale. This STD is caused by the bacterium Calymmatobacterium granulomatis. It is characterized by the appearance of lesions in the genital or anal area. Granuloma inguinale is most common in southeast India, Guyana and New Guinea. It rarely occurs in the United States, with only about 100 cases being reported each year. According to the NIH, men are infected nearly twice as often as women, and most infections occur among people between the ages of 20 and 40. Anal intercourse is considered the most frequent source of transmission. Complications may include destruction and scarring of the genitals, genital depigmentation (loss of pigment in the genital area) and permanent genital swelling. In some cases, the disease may spread throughout the body. This may result in death from secondary complications including heart failure, pneumonia or hemorrhage. If treated early, complications can be avoided.