The Most Common STDs: Human Papillomavirus (HPV).

What it is: A viral infection with more than 100 different strains, 30 of which are sexually transmitted and can infect the genital area with warts, both inside and out. Some other types of HPV may cause abnormal Pap tests, and can lead to certain cervical or other kinds of cancers.

How many get it: An estimated 6.2 million new cases each year. At least 20 million people already have it.

Symptoms: Soft, itchy warts in and around the genitals (vagina, penis, testicles, anus) may appear weeks or months after exposure. Many people, however, have no symptoms but are still contagious. Cervical changes that may indicate cancer-causing cells can only be detected by Pap tests.

How It's Spread: Through unprotected vaginal, oral, or anal sex, or by touching an infected area (though infected areas aren't always noticeable).

Testing: If you have warts, a doctor can diagnose you visually. A Pap test can show abnormal results that may be a sign of HPV.

Treatment: There is no cure for HPV, but the genital warts can be treated in several ways, depending on their size and location. Recently, an HPV vaccine was approved for girls/women ages 9-26 years to prevent cervical cancer and other diseases caused by certain types of HPV. The vaccine protects against four HPV types, which are responsible or 70% of cervical cancers and 90% of genital warts.

Possible Consequences: Increased risk for infection of other STDs, including HIV. In women, the persistent infection with certain HPV types is the most important risk factor for cervical cancer; however, the majority of infected women do not develop cervical cancer.

Most Common STDs: Chlamydia

What Is It: A bacterial infection of the genital area.

How Many Get It: About 3 million new cases each year. The highest rates are among women aged 15 to 19.

Symptoms: There are no symptoms in most women and many men who have it. Others may experience abnormal vaginal bleeding (not your period), unusual discharge or pain during urination within one to three weeks of having sex with an infected partner.

How It's Spread: Through unprotected vaginal, oral or anal intercourse.

The Test: Swabs are used to collect and test discharge from the vagina or penis, urine tests are also available.

Treatment: Oral antibiotics cure the infection. Both partners must be treated at the same time to prevent passing the infection back and forth, and both partners need to abstain from intercourse until the infection is gone.

Possible Consequences: Infertility and increased risk of HIV infection. In women, untreated chlamydia can lead to pelvic inflammatory disease (PID) and tubal (ectopic) pregnancy.

Most Common STDs: Trichomoniasis

What Is It: A parasitic infection of the genital area.

How Many Get It: An estimated 7.4 million new cases each year.

Signs: Often there are no symptoms, especially in men. Some women notice a frothy, smelly, yellowish-green vaginal discharge and/or genital area discomfort, usually within 5 to 28 days after exposure to the parasite. Men may notice a discharge from the penis or a slight burning after urination or ejaculation.

How It's Spread: Through unprotected vaginal intercourse or vulva-to-vulva contact with an infected partner. Women can acquire the disease from infected men or women, but men usually contract it only from infected women.

The Test: Swabs are used to collect and test discharge from the vagina or penis.

Treatment: Antibiotics can cure the infection. Both partners have to be treated at the same time to prevent passing the infection back and forth and need to abstain from intercourse until the infection is gone.

Possible Consequences: Increased risk for infection of other STDs, including HIV. In women, trich can cause complications during pregnancy. Also, it's common for this infection to happen again and again.

Most Common STDs: Gonorrhea

What Is It: A bacterial infection of the genital area.

How Many Get It: Approximately 700,000 new cases a year. The highest rates are among women aged 15 to 19 and males between the ages of 20 to 24.

Symptoms: Most people infected have no symptoms. For those who do, it can cause a burning sensation while urinating, green or yellowish vaginal or penile discharge, anal discharge or discomfort, and for women, abnormal vaginal bleeding and/or pelvic pain. Symptoms can appear within 2 to 30 days after infection.

How It's Spread: Through unprotected vaginal, oral, or anal sex.

The Test: Swabs used to collect and test discharge from the vagina, penis, anus or mouth, urine test also available.

Treatment: Oral antibiotics. Both partners must be treated at the same time to prevent passing the infection back and forth, and both partners need to abstain from intercourse until the infection is gone.

Possible Consequences: Increased risk of infection of other STDs, including HIV, and increased risk of infertility. In women, the infection can spread into the uterus and fallopian tubes, causing PID, which can lead to infertility and tubal (ectopic) pregnancy.

Most Common STDs: Genital Herpes

What Is It: A viral infection of the genital or rectal area that also can occur around the mouth.

How Many Get It: About 1 million new cases each year. An estimated 45 million cases already exist.

Signs: Most people have no symptoms. Herpes 1 causes cold sores and fever blisters on the mouth but can be spread to the genitals; Herpes 2 is usually on the genitals, but can be spread to the mouth. Outbreaks are often recurrent and can cause red bumps that turn into painful blisters or sores on genitals and elsewhere. During the first attack, flu-like symptoms-fever, headaches, and swollen glands-may occur. Symptoms usually appear within 2 weeks of infection and can take 2 to 4 weeks to heal.

The Test: During a herpes outbreak, a cotton swab can be used to test sores or lesions for the presence of the virus. Blood tests are also available.

How It's Spread: By touching an infected area (infected areas may not always be noticeable), or having unprotected vaginal, oral, or anal intercourse. Warning: Some people may be contagious even when they don't have symptoms.

Treatment: There is no cure, but medications can help reduce the pain, itching and frequency of recurrent outbreaks as well as reduce transmission to partners.

Possible Consequences: Increased risk for infection of other STDs, including HIV. Recurring sores--the virus lives in the nerve roots and keeps coming back. Passing herpes from mother to newborn is rare; however, an infant who gets herpes can become very ill, so precautions are advisable.

Most Common STDs: Syphilis

What Is It: A bacterial infection that can spread throughout the body.

How Many Get It: About 70,000 new cases a year.

Signs: In the first phase, a single sore (chancre) may appear on the genitals or mouth 10 to 90 days after exposure and may last for three to six weeks. Often, however, there are no noticeable symptoms. In the second stage, up to several weeks after the first sore has disappeared, a variety of symptoms can appear, including a rash (often on the palms of the hands, soles of the feet, or genital area). Though symptoms may disappear, the disease is still present.

How It's Spread: Through unprotected vaginal, oral, or anal sex--and also through kissing if there is a lesion on the mouth.

The Test: A blood test is available, or test of open sores.

Treatment: Antibiotic treatment can cure the disease if it's caught early, but medication can't undo damage already done. Both partners must be treated at the same time and must abstain from intercourse until the sores are completely healed.

Possible Consequences: Increased risk for infection of other STDs, including HIV. Untreated, the symptoms will disappear, but the infection stays in the body and can progress into the third stage, damaging the brain, heart, and nervous system, and possibly causing death. Syphilis in women can seriously harm a developing fetus during pregnancy.

Most Common STDs: Hepatitis B Virus (HBV)

What Is It: A viral infection affecting the liver.

How Many Get It: About 73,000 new cases a year.

Signs: Many people don't have any symptoms. Others may experience tiredness, aches, nausea and vomiting, loss of appetite, darkening of urine, or tenderness in the stomach, usually within one to six months of exposure. Yellowing of the skin and the whites of the eyes (called jaundice) can occur later.

How It's Spread: Through unprotected vaginal, oral, and anal sex, but also through sharing contaminated needles or any behavior in which a person's mucus membranes are exposed to an infected person's blood, semen, vaginal secretions, or saliva. The chance of getting it through kissing is low.

The Test: A blood test.

Treatment: Most cases clear up within one to two months without treatment, during which time you should not drink alcohol until liver function returns to normal. Some people are contagious for the rest of their lives. A three-dose vaccine is now available.

Possible Consequences: Increased risk for infection of other STDs, including HIV, as well as Hepatitis C. Chronic, persistent inflammation of the liver and later cirrhosis or cancer of the liver. If you are infected and pregnant, your baby must be immunized at birth.

Most Common STDs: HIV

What Is It: The human immunodeficiency virus (HIV), the cause of AIDS.

How Many Get It: An estimated 40,000 people in the U.S. are infected each year, the majority through sexual transmission. Almost 1 million people in the U.S. are currently living with HIV.

Signs: Many people who have HIV don't know it because symptoms may not appear for 10 years or longer. Others experience unexplained weight loss, flu-like symptoms, diarrhea, fatigue, persistent fevers, night sweats, headaches, or severe or recurring vaginal yeast infections.

How It's Spread: Through unprotected vaginal, oral, and anal sex, through sharing contaminated needles, and during pregnancy and breast-feeding. During penetrative intercourse, the receptive partner is at higher risk for HIV infection than is the insertive partner.

The Test: Oral and blood tests available.

Treatment: There is no cure for AIDS, and it is considered fatal. Antiviral medications can slow the progression of HIV infection and delay the onset of AIDS symptoms. Early treatment can make a big difference.

Possible Consequences: Increased risk for infection of other STDs. HIV is the deadliest STD of all and can weaken the body's ability to fight disease, making one vulnerable to certain cancers and infections such as pneumonia.