Nearly half of all STIs occur in people ages 15 to 24, according to the CDC. But other age groups are not immune. Whenever people have unprotected sex with multiple partners, or the partner of a monogamous person has sex with multiple people, the risk increases for STIs. These diseases affect men and women of all racial backgrounds, ages, and economic levels.

Some STIs are much lower in number than they have been, but the overall number of STIs is rising, the National Institute of Allergy and Infectious Diseases (NIAID) says. This is mostly because young people have become sexually active at an earlier age, yet they are marrying later. Divorce is also 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 life. Having multiple sex partners is a major risk factor for getting STIs. Research has also found that people with an STD are 2 to 5 times more likely to get HIV, the virus that causes AIDS. They are also more likely to pass HIV on to others.

Most STIs cause symptoms for only a short time. Or they cause no symptoms at all. If a person has had multiple sex partners during their life, they are at risk of getting an STI. If a person is infected with an STI, they can pass it on to a sex partner—even if no symptoms are present. If you have more than one sex partner, or if your partner has had more than one partner, experts recommend that you get screened for STIs.

Health problems caused by STIs tend to be more serious and happen more often in women than in men. This is because women are more likely to get HIV, gonorrhea, and chlamydia. They also may not know they are infected until serious problems have developed. PID occurs when STIs spread into the uterus and fallopian tubes. PID can cause infertility and ectopic pregnancy. The human papillomavirus (HPV) infection, which causes genital warts, can cause cervical cancer and other cancers of the reproductive system. A pregnant woman with an STD can pass the disease on to her baby. Some of these illnesses can cause permanent disabilities or death.

About 2.8 million new cases of chlamydia are reported each year, according to the NIAID. Untreated chlamydia can lead to PID in women. But most people with chlamydia don't have symptoms. Chlamydia is treated with antibiotics. Because so many people have chlamydia, the CDC recommends that sexually active female teens and young women, as well as older women at risk, be screened every year for this STI. The CDC also recommends that women who are treated for chlamydia be rescreened 3 to 4 months after treatment.

Genital herpes is caused by the herpes simplex virus (HSV). It is a viral infection that has no cure. The blisters or sores in the genital area usually go away within a few days or up to 2 weeks, but the virus stays in the body. The sores or blisters may come back from time to time. A tingling or burning in the legs, buttocks, or genital area usually happens just before the blisters show up. Medicines are available to treat and prevent genital herpes outbreaks and help prevent passing it on to others. But these medicines don't get rid of the virus.

The CDC recommends only one class of antibiotics to treat gonorrhea—the cephalosporins. The most common symptom of gonorrhea is a discharge from the vagina or penis and painful or difficult urination. Women with gonorrhea can develop PID, ectopic pregnancy, and infertility. Overall, STIs that are caused by bacteria are becoming resistant to antibiotics.

Although the first symptoms of syphilis are mild, this STD eventually can be fatal if it isn't treated. The first symptom is a painless sore called a chancre that can show up near or on the genitalia, the mouth or anus, or on the hands. It can take years for syphilis to become advanced. It involves the heart and central nervous system. Syphilis is most often treated with penicillin.

Chancroid (pronounced SHAN-kroid) is an STI caused by bacteria. It occurs in the U.S. in outbreaks from time to time, but it is common in sub-Saharan Africa. Like genital herpes and syphilis, the first symptom of chancroid is an open sore on the genitals. Lymph nodes in the groin swell and become painful. Scabies is a skin infection with a tiny mite. It is fairly common and highly contagious. It can be spread through sexual contact. But it also can be passed on by nonsexual contact with skin, infested sheets, towels, and furniture. Scabies causes intense itching, especially 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. In adults, the virus is passed on sexually. Symptoms are bumps on the chest, 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 U.S. It is mostly found among IV drug users and people who have genital ulcers or a history of syphilis. HIV is also usually passed sexually.

CMV is a member of the herpesvirus family. It is spread not only by sexual contact, but also by other forms of contact. 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 of getting CMV. The virus doesn't spread easily. Casual contact with an infected person doesn't usually pass on the infection. Like other herpesviruses, CMV has no cure. But the virus usually stays inactive in the body.

Other ways include:

• Don't have a partner who has other sexual partners.
• Don't have sex with people who have, or who you think have, an STI.

The best way to not get an STI is to not have sexual contact with others. It is also important to get tested and treated for STIs to protect yourself and others.