Eunice Kennedy Shriver National Institute of Child Health and Human Development

STDs/STIs are a group of illnesses that are passed from person to person during sexual intercourse, oral sex, or certain types of sex play. These diseases can be caused by bacteria, viruses, or parasites and are spread through intimate sexual contact involving the penis, vagina, mouth, or anus or contact with any of the membranes that line the urinary and/or genital tracts. STDs/STIs are also called venereal diseases.

Health care providers often use the term "infection" rather than "disease" because it is possible for a person to have no symptoms but still have the infection and require treatment. Scientists have identified more than 20 different STIs.¹

Learn more about STDs from the Centers for Disease Control and Prevention website.

People with STDs/STIs may feel ill and notice some of the following signs and symptoms:^1,2

• Unusual discharge from the penis or vagina
• Sores or warts on the genital area
• Painful or frequent urination
• Itching and redness in the genital area
• Blisters or sores in or around the mouth
• Abnormal vaginal odor
• Anal itching, soreness, or bleeding
• Abdominal pain
• Fever

In some cases, people with STIs have no symptoms. Over time, any symptoms that are present may improve on their own. It is also possible for a person to have an STI with no symptoms and then pass it on to others without knowing it.

If you are concerned that you or your sexual partner may have an STI, talk to your health care provider. Even if you do not have symptoms, it is possible you may have an STI that needs treatment to ensure your and your partners’ sexual health.

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), and Zika
• Parasites, such as trichomonas vaginalis, or insects such as crab lice or scabies mites¹

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.

Approximately 20 different infections are known to be transmitted through sexual contact. Although NICHD does study STIs, their prevention, and their effects on pregnancy and long-term health, the Institute is not the lead agency aiming to understand STIs. For more complete information about STIs, you may want to visit http://www.cdc.gov/std/default.htm or https://www.niaid.nih.gov/diseases-conditions/sexually-transmitted-diseases.

Here are descriptions of some common STIs.

Some infections—such as Zika, gonorrhea, chlamydia, HIV, and syphilis—can pass to the fetus during pregnancy or to the infant during delivery, causing short- and long-term health problems. However, the risk of transmission can be lowered or even eliminated with appropriate treatments.

For this reason, it is important for a pregnant woman to be tested for STDs/STIs as a part of her prenatal care. Prenatal STI testing can determine whether a pregnant woman has an infection that can be cured or controlled with drug treatment, which decreases the chances that the infant will contract the disease.¹ The pregnant woman and her health care provider can take other steps to protect her health and her infant's health if the STI is one that cannot be cured or treated.

Some examples of ways to reduce or eliminate risk of transmission include the following:

• HIV can be passed from mother to infant during pregnancy before birth, at the time of delivery, or after birth during breastfeeding.² Treatment during pregnancy near delivery prevents this transmission.
• Recent research also shows that, for HIV-infected mothers whose immune system is in good health, treatment during breastfeeding virtually eliminates transmission of HIV through breastmilk.³ Women who have HIV but whose immune systems are not in good health should not breastfeed their infants if safe alternatives, such as infant formula, are available.¹
• If a woman has active herpes lesions, untreated HIV, or an HIV viral load that is not suppressed, the infant can be delivered by cesarean section (also referred to as C section) to prevent transmission of the infection.
• For infections, such as gonorrhea, a pregnant woman and her sexual partner can be treated before the birth, and the infant can be treated at birth to prevent infection.¹
• In most hospitals, infants' eyes are routinely treated with an antibiotic ointment shortly after birth. The ointment can prevent blindness from exposure to gonorrhea or chlamydia bacteria during delivery, in case the pregnant woman had an undetected infection.
• Women who are pregnant or are thinking about getting pregnant are advised not to travel to areas where Zika infection has been reported. Preventing mosquito bites can also reduce the risk for infection and transmission.

STIs during pregnancy can also cause:²

• Miscarriage (fetal loss before 20 weeks)
• Ectopic pregnancy (when the embryo implants outside of the uterus, usually in a fallopian tube)
• Preterm labor and delivery (before 37 completed weeks of pregnancy)
• Low birth weight
• Congenital anomalies, including blindness, microcephaly, deafness, bone deformities, and intellectual disability
• Stillbirth (fetal loss at or after 20 weeks)
• Illness in the newborn period (first month of life)
• Newborn death
• Health complications in the mother

Visit the What infections can affect pregnancy? section of this website for more information.

Any person who is sexually active should discuss his or her risk factors for STDs/STIs with a health care provider and ask about getting tested. If you are sexually active, it is important to remember that you may have an STD/STI and not know it because many STDs/STIs do not cause symptoms. You should get tested and have regular checkups with a health care provider who can help assess and manage your risk, answer your questions, and diagnose and treat an STD/STI if needed.

Starting treatment quickly is important to prevent transmission of infections to other people and to minimize the long-term complications of STDs/STIs. Recent sexual partners should also be treated to prevent re-infection and further transmission.

Some STDs/STIs may be diagnosed during a physical exam or through microscopic examination of a sore or fluid swabbed from the vagina, penis, or anus. This fluid can also be cultured over a few days to see whether infectious bacteria or yeast can be detected. The effects of human papilloma virus (HPV), which causes genital warts and cervical cancer, can be detected in a woman when her health care provider performs a pap smear test and takes samples of cells from the cervix to be checked microscopically for abnormal changes.¹ Blood tests are used to detect infections such as hepatitis A, B, and C or HIV/AIDS.

Because sexually transmitted diseases are passed from person to person and can have serious health consequences, the health department notifies people if they have been exposed to certain STDs/STIs. Not all STDs/STIs are reported, though. If you receive a notice, it is important to see a health care provider, be tested, and start treatment right away.

Screening is especially important for pregnant women, because many STDs/STIs can be passed on to the fetus during pregnancy or delivery. During an early prenatal visit, with the help of her health care provider, an expectant mother should be screened for these infections, including HIV  and syphilis. Some of these STDs/STIs can be cured with drug treatment, but not all of them. However, even if the infection is not curable, a pregnant woman can usually take measures to protect her infant from infection.²

Viruses such as HIV, genital herpes, human papillomavirus, hepatitis, and cytomegalovirus cause STDs/STIs that cannot be cured. People with an STI caused by a virus will be infected for life and will always be at risk of infecting their sexual partners. However, treatments for these viruses can significantly reduce the risk of passing on the infection and can reduce or eliminate symptoms. STIs caused by bacteria, yeast, or parasites can be cured using appropriate medication.

STDs/STIs caused by bacteria or parasites can be treated with antibiotics. These antibiotics are most often given by mouth (orally). However, sometimes they are injected or applied directly to the affected area.

The treatments, complications, and outcomes for viral STIs depend on the particular virus (HIV, genital herpes, human papillomavirus, hepatitis, or cytomegalovirus). Treatments can reduce the symptoms and the progression of most of these infections. For example, medications are available to limit the frequency and severity of genital herpes outbreaks while reducing the risk that the virus will be passed on to other people.

Individuals with HIV need to take special antiretroviral drugs that control the amount of virus they carry. These drugs, called highly active antiretroviral therapy, or HAART,¹ can help people live longer, healthier lives and can prevent onward transmission of HIV to others. If a woman with HIV becomes pregnant, these medicines also can reduce the chance that her fetus or infant will get the infection.

The Types and Treatments page of this website provides some general information about treatments for certain STIs. The Centers for Disease Control and Prevention website also provides more specific information about treatments for STIs.

Getting tested and treated for STIs is especially important for pregnant women because some STIs may be passed on during pregnancy or delivery. Testing women for these STIs early in their pregnancy is important so that steps can be taken to help ensure delivery of a healthy infant. The necessary treatment will depend on the type of STI involved.

Whatever the infection, and regardless of how quickly the symptoms resolve after beginning treatment, the infected person and their partner(s) must take all of the medicine prescribed by the health care provider to ensure that the STI is completely treated. Likewise, they should follow health care provider recommendations about how long to abstain from sex after the treatment is completed to avoid passing the infection back and forth.

NICHD research on STDs/STIs ranges from identifying factors and behaviors that affect the risk of contracting or spreading STDs/STIs to understanding how STDs/STIs affect pregnancy and long-term health to developing and testing new interventions to prevent the spread of STDs/STIs in different populations. Also, since 1987, NICHD has supported research and training activities that have helped to establish and define the field of contraceptive microbicides, products that both prevent pregnancy and reduce the transmission of STIs, including HIV.

NICHD-supported researchers are also trying to understand the best ways to communicate with people about STIs and effective preventive measures. Some of this research includes studying attitudes, perception, and knowledge of STIs and their prevention; understanding the use and misuse of contraception to prevent STIs; and understanding the misinformation surrounding the topic while aiming to identify the best settings for providing education about sexual health.

Because preventing and treating STIs is a major goal for NIH and its Institutes, NICHD conducts and supports a variety of clinical trials on STIs, including HIV.