There are three major causes of STDs/STIs:

• Bacteria, including chlamydia, gonorrhea, and syphilis.
• Viruses, including HIV/AIDS, herpes simplex virus, human papillomavirus, hepatitis B virus, cytomegalovirus (CMV).
• Parasites, such as trichomonas vaginalis, or insects such as crab lice or scabies mites.

Any STI can be spread through sexual activity including sexual intercourse, and some STIs also are spread through oral sex and other sexual activity. Ejaculation does not have to occur for an STI to pass from person to person.

In addition, sharing contaminated needles, such as those used to inject drugs, or using contaminated body piercing or tattooing equipment also can transmit some infections, such as HIV, hepatitis B, and hepatitis C. A few infections can be sexually transmitted but are also spread through nonsexual, close contact. Some of these infections, like CMV, are not considered STIs even though they can be transmitted through sexual contact.

Regardless of how a person is exposed, once a person is infected by an STI, he or she can spread the infection to other people through oral, vaginal, or anal sex, even if he or she has no symptoms.

Signs and symptoms may include:

• Clear, white, greenish or yellowish vaginal discharge: Yellow discharge is often a sign of an infection. You should see a doctor if you have yellow discharge before your period, especially if: the discharge has a strong smell. The discharge is chunky or frothy.

• Discharge from the penis: Penile discharge may be watery (clear) or cloudy (containing pus). The discharge may also be bloody. Urinary tract infection (UTI) or a sexually transmitted infection are common causes of penile discharge. Penile discharge is often accompanied by pain or burning during urination and a need to urinate frequently. Itching can also accompany penile discharge. Chlamydia, gonorrhea, and Trichomonas infections are common infections that may cause penile discharge.

• Strong vaginal odor: Vaginal odor is any unpleasant odor that comes from the vagina. It’s normal for your vagina to have its own unique smell, but a very strong odor may signal a problem.

• Vaginal itching or irritation: Vulvar and vaginal itching are common. There are several possible causes, including irritation, yeast infections, and certain skin conditions. The vulva is the outer part of female genitals, including the lips, clitoris, bladder opening, and vaginal opening.

• Itching or irritation inside the penis: People may experience itching on the shaft, tip, or all over the penis. The itch may occur for several reasons, including STIs, bacterial infections, skin conditions, yeast infections, fungal infections, and shaving. Home remedies may provide some relief from penis itching.

• Pain during sexual intercourse

• Painful urination: Painful urination can be a symptom of certain sexually transmitted infections, including chlamydia and gonorrhea, but it can also be a sign of a urinary tract infection.