Nearly two-thirds of all STDs occur in people under 25 years of age, the NIAID says. Other age groups are not immune, however. Whenever people have unprotected sex with multiple partners, or the partner of a monogamous person has sex with multiple people, the risk increases for STDs. These diseases affect men and women of all backgrounds and economic levels.

Although infection by some STDs has fallen significantly, the overall incidence of STDs is rising, the NIAID says, mostly because young people have become sexually active at an earlier age, yet they are marrying later. Also, divorce is more common today than it was in the past. These factors mean that sexually active people are more likely to have several sexual partners during their lives. Having multiple sex partners is a major risk factor for developing STDs. Research has also found that people with an STD are two to five times more likely to contract HIV, the virus that causes AIDS, and that they are more likely to pass HIV on to others.

Most STDs cause no symptoms. If a person has had multiple sex partners during his or her life, he or she is at risk for developing an STD. If a person is infected with an STD, he or she can pass it on to a sex partner -- even if no symptoms are present. If you have more than one sex partner, experts recommend that you get screened for STDs.

Health problems caused by STDs tend to be more severe and more frequent for women than for men, the NIAID says. This is because women are more susceptible to HIV, gonorrhea and chlamydia, and because they may not know they are infected until serious problems have developed. Pelvic inflammatory disease occurs when STDs spread into the uterus and fallopian tubes. PID can lead to infertility and ectopic pregnancy. The human papillomavirus infection (HPV), which causes genital warts, has been linked to cervical cancer and other cancers of the reproductive system. In addition, a pregnant woman with an STD can pass the disease on to her baby. Some of these illnesses can cause permanent disabilities or death.

Up to 8 million new cases of chlamydia are reported each year, according to the NIAID. Untreated chlamydia can lead to PID in women. Most people with this infection have no symptoms, however. It is treated with antibiotics. Because of the widespread incidence of chlamydia, the CDC now recommends that sexually active female teens and young women, as well as older women at risk, be screened annually for this STD. The CDC also recommends that women who are treated for chlamydia be rescreened three to four months after treatment.

Genital herpes is caused by the herpes simplex virus (HSV) and is an incurable viral infection. The blisters or sores in the genital area usually disappear within two to three weeks, the NIAID says, but the virus remains in the body. The sores or blisters may reappear from time to time, often preceded by a tingling or burning in the legs, buttocks or genital area. Although medications are available to treat the symptoms of genital herpes, these drugs do not get rid of the virus. Other STDs caused by viruses also are incurable.

Because of this, new antibiotics or combinations of antibiotics must now be used to treat gonorrhea that doesn't respond to penicillin. The most common symptom of gonorrhea is a discharge from the vagina or penis and painful or difficult urination, the NIAID says. Women with gonorrhea can develop PID, ectopic pregnancy and infertility. Overall, STDs that are caused by bacteria are becoming resistant to antibiotics.

Although the initial symptoms of syphilis are mild, if untreated, this STD eventually can be fatal. The first symptom is a painless sore called a chancre that can appear near or on the genitalia, the mouth or anus, or on the hands. Advanced syphilis, which can take years to reach, can involve the heart and central nervous system. Syphilis is treated with penicillin.

Chancroid (pronounced SHAN-kroid) is a bacterial STD. It occurs in the United States in periodic outbreaks, but it is common in sub-Saharan Africa. Like genital herpes and syphilis, the initial symptom of chancroid is an open sore on the genitals; lymph nodes in the groin become painful. Scabies is a skin infection with a tiny mite. It is fairly common and highly contagious. It is spread primarily through sexual contact, but also can be passed on by contact with skin, infested sheets, towels and furniture, the NIAID says. Scabies causes intense itching, particularly at night. Small red bumps or lines appear on the body where the female scabies mite has burrowed into the skin to lay her eggs. Molluscum contagiosum is a common viral infection that most often affects young children, who pass it to one another through saliva, the NIAID says. In adults, the virus is transmitted sexually; symptoms are sores on the genitals, lower abdomen, buttocks or inner thighs. Human T-cell Lymphotropic Virus (HTLV) can be passed from one person to another not only through sexual contact, but also through blood. This is a rare disease in the United States, found primarily among intravenous drug users and people who have genital ulcers or a history of syphilis, the NIAID says.

CMV is a member of the herpesvirus family and is spread not only by sexual contact, but also by other forms of physical contact, the NIAID says. The virus is found in saliva, urine and other bodily fluids. Because the virus is also often found in semen, as well as in secretions from the cervix, it can be spread by sexual contact. People in a day-care setting who care for children 3 and younger are at higher risk for contracting CMV. Like other herpesviruses, CMV is incurable. The virus, however, usually remains inactive in the body.

Other ways include not having a partner who has other sexual partners; not having sex with intravenous drug users; and not having sex with people who have, or whom you suspect have, an STD. The best way to avoid developing an STD is to avoid sexual contact with others.