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. Although 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 Unites States each year.
  
  
• Trichomoniasis. This STD 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 the most common curable STD in young women. Trichomoniasis increases the risk of human immunodeficiency virus (HIV) transmission.
  
  
• Scabies. This STD 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 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 and vaginal fluids. In addition to being transmitted sexually, HIV may also be spread by sharing needles used to inject intravenous drugs, during blood transfusions, or from mother to child (during delivery or breastfeeding). According to the CDC, approximately 1 million people in the United States are currently infected with HIV (which impairs the body’s ability to fight infection)/AIDS, with 24 to 27 percent undiagnosed and unaware of their HIV infection. People with AIDS are highly susceptible to many life-threatening diseases and certain forms of cancer.
  
  
• Genital herpes. This STD is caused by the herpes simplex virus (HSV). The two types of HSV include type 1 (HSV-1) and type 2 (HSV-2). Both types can cause genital herpes but most cases are caused by HSV-2. The viruses are 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. Performing oral sex while having an active cold sore can cause transmission of the virus to the partner’s genital area. Although rare, genital herpes can also be transmitted through contact with a toilet seat or hot tub. 
  
  Although many people do not experience any signs or symptoms, genital herpes is often 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). This STD 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, patients may develop small, flesh-colored bumps. HPV can infect the vaginal lining, vulva, cervix, anus, rectum, mouth and throat. It can also infect the penis and scrotum in men. Certain types of HPV may also cause cervical dysplasia (precancerous changes in the cervix). According to the CDC, at least 80 percent of women will have acquired the infection by the age of 50 years and most sexually active people will acquire the infection at some point during their lifetime. Although most people are exposed to the cell-changing types of HPV at some point in their lives, not everyone will develop abnormal cell changes. 
  
  
• 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 fetus during vaginal childbirth. In women, the bacterium 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. This STD is caused by the bacterium Neisseria gonorrhea, which can easily grow and multiply in warm moist areas of the body, including a woman’s cervix, uterus and fallopian tubes. It can also grow in the urethra, mouth, throat, eyes and anus. In some cases, it 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 baby 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 a syphilis sore. 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. Rates are declining among African Americans and women. 
  
  
• Chancroid. This STD is caused by the bacterium Haemophilus ducreyi. Chancroid is primarily found in developing and third world countries in Africa and Asia. Generally it is 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. Outbreaks most often occur among minority populations in the inner cities, especially in the southern and eastern sections of the country. 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 National Institutes of Health, men are infected nearly twice as often as women, and most infections occur among people between the ages of 20 and 40 years. Anal intercourse is considered the most frequent source of transmission. Complications of this STD 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.