An STI--short for sexually transmitted infection--is an infection you can get by having sex. This includes intercourse, anal sex, oral sex or skin-to-skin contact. Some of the most common STIs are listed below.

The only 100% sure way to protect yourself is not have sex. It only takes one sexual contact with someone who has an STI to catch it. If you are sexually active, protect yourself by using a latex condom every time you have sex. But know that condoms can't prevent you from coming into contact with some sores, such as those that can occur with herpes or genital warts.

Talk to your doctor if you are thinking about using a spermicide. Research has shown that spermicides containing nonoxynol-9 can cause genital irritation and may increase your risk of catching an STI. However, using a condom with nonoxynol-9 is better than not using a condom at all.

Symptoms: Girls and guys may have redness and itching around the genitals.

Treatment: Medicine, such as over-the-counter lotions os shampoos (some brand names: Nix, Rid) can kill the lice. Prescription shampoos, lotions or pills are also available to treat pubic lice if over-the-counter medicine doesn't work. However, lice can come back if clothes, sheets and towels aren't washed. Usually both partners need to be treated for pubic lice.

Symptoms: Girls can have a heavy, greenish-yellow frothy discharge and pain when urinating or while having sex. In girls, trichomoniasis can also cause redness, itching and a burning feeling in the genital area. Guys may also have burning with urination or ejaculation.

Treatment: Antibiotics. Usually both partners need to be treated.

Symptoms: Girls may have no symptoms or may have pain when urinating, itching around the vagina, yellow fluid (discharge) from the vagina, bleeding between periods or pain in the lower abdomen. Guys may have a burning sensation when urinating and a milky colored discharge from the penis. If not treated, chlamydia can cause infertility and other problems in girls and painful swelling of the scrotum in guys.

Treatment: Antibiotics. Usually both partners should be treated.

Symptoms: An early symptom is a red, painless sore, called a chancre (say: shang-ker). The sore can be on the penis, vagina, rectum, tongue or throat. The glands near the sore may be swollen. After a few months, both guys and girls can get a fever, sore throat, headache or pain in their joints. Another symptom is a scaly rash on the palms of the hands or the bottom of the feet. The sores and other symptoms go away, but this does not mean that the infection is gone. Syphilis can cause serious health problems if it's not treated.

Treatment: Antibiotics. If one partner is infected, the other should be tested.

Symptoms: HIV (human immunodeficiency virus) causes AIDS. HIV makes the body's immune system weak so it can't fight disease. Symptoms may take years to develop, and can include infections, feeling tired for no reason, and night sweats.

Treatment: Medicines can treat symptoms but can't cure AIDS. If one partner is infected, the other should be checked by a doctor.

Symptoms: Girls and guys may have tingling, pain or itching around the vagina or penis. Small blisters can form in these areas and then break open. When they break open, the sores can cause a burning feeling. It may hurt to urinate. Some people have swollen glands, fever and body aches. The sores and other symptoms go away, but this does not mean that the infection is gone. The sores and blisters can come back (called an "outbreak").

Treatment: Medicine can treat symptoms but can't cure herpes. If one partner is infected, the other should by checked by a doctor.

Symptoms: Girls may have no symptoms or may have white, green, yellow or bloody discharge from the vagina, pain when urinating, bleeding between periods, heavy bleeding during a period or a fever. Both girls and guys can get sore throats if they've had mouth to penis or vagina contact (oral sex). Guys may have thick, yellow discharge from the penis and pain when urinating. The opening of the penis may be sore. Gonorrhea can cause serious health problems if it's not treated.

Treatment: Antibiotics. Usually both partners should be treated.

Symptoms: HPV (human papillomavirus) can cause warts in or around the vagina, penis or rectum. In girls, the warts can be inside the body on the cervix or vagina so you can't see them. Or they may be on the outside of the body, but may be too small to see. The warts usually don't hurt. There are many types of HPV. Depending on the type, HPV may not cause any symptoms.

Treatment: No medicine cures HPV. A doctor can remove external warts. Warts on the cervix or in the vagina can cause changes that may lead to cervical cancer. Doctors will watch these changes. If one partner is infected with HPV, the other should be checked by a doctor.

Some types of HPV can be prevented. There is a vaccine that can prevent 4 different types of HPV in young women. This vaccine targets the types of HPV that cause up to 70% of all cases of cervical cancer and about 90% of all cases of genital warts. The Centers for Disease Control and Prevention's (CDC) Advisory Committee on Immunization Practices (ACIP) recommends that girls between the ages of 11 and 12 receive the vaccine. The vaccine is also approved for boys and girls and men and women between the ages of 9 years and 26 years.

Ask. Although it may be uncomfortable, talk to your partner before having any sexual contact. Ask if he or she is at risk for having an STI. Some of the risk factors are having sex with several partners, using injected drugs and having had an STI in the past. To be safe, protect yourself no matter what the person says. You must also tell your partner if you have an STI. You aren't doing yourself or your partner any favors by trying to hide it.

Watch for the symptoms listed above, but remember that most STIs don't cause any symptoms.

If you are sexually active, tell your doctor and get regular check-ups. If you're worried that your parents will find out, you can ask your doctor not to discuss it with your parents. You can also go to a free health clinic. Don't let fear keep you from getting checked out. Imagine how you'd feel--and how your parents would feel--if you got really sick because you didn't get help. If you find out you have an STI, both you and your partner should get treated right away.

Some can. STIs that are caused by bacteria, like chlamydia, can be cured with antibiotics. But STIs caused by a virus (like HIV or herpes) can't be cured. Your doctor can only treat the symptoms that the virus causes.

Don't wait to be treated! Early treatment helps prevent serious health problems. Even if medicine can't completely cure the STI, it can help keep you from getting really sick. If you are given medicine for an STI, take it exactly as the doctor says.