What are sexually transmitted diseases?

Sexually transmitted diseases (STDs) are also sometimes called sexually transmitted infections (STIs); indeed, the terms are often used interchangeably. Many public health experts prefer the term “infection” because “disease” suggests a clear medical problem with obvious signs or symptoms, which, according to the American Sexual Health Association, isn’t always the case with such infections. In other words, the viruses or bacteria that cause these infections don’t always cause disease.

Many common sexually transmitted infections spread through oral, anal or vaginal intercourse and genital touching. In order for an STI to be passed on, one partner must have the infection. If left untreated, STIs can lead to increased risk for other sexually transmitted infections, especially HIV. Twenty million new STI infections are diagnosed in the United States each year.

Who is at risk for STIs?

Anyone who engages in sexual contact of any kind is at risk for STIs. Age, sexual preference or gender does not matter. According to the Centers for Disease Control and Prevention (CDC), young people between ages 15 and 24 account for half of all new STI diagnoses annually. Men who have sex with men regardless of sexual identity are also disproportionately affected by STIs. These viruses or bacteria can also infect women who are pregnant and their unborn babies.

What are some common STIs?

There are about 30 different types of STIs. Some common ones include those listed below:


Chlamydia is a bacterial infection transmitted via semen, pre-cum and vaginal fluids. It is one of the most commonly reported STIs in the United States, meaning that when the illness is diagnosed local health departments must be notified. The infection may be hard to detect at first, as there are sometimes few or no symptoms in the early stages.

Signs and symptoms may include:

  • Painful urination
  • Lower abdominal pain
  • Vaginal discharge in women
  • Discharge from the penis in men
  • Pain during sexual intercourse in women
  • Bleeding between periods in women
  • Testicular pain in men
  • Rectal pain, bleeding or discharge.

The STI is most prevalent in young women. Chlamydia may cause permanent damage, such as pelvic inflammatory disease (infection and inflammation of a woman’s pelvic organs, including the uterus, Fallopian tubes, ovaries and cervix). Additionally, this STI may make it difficult to become pregnant or lead to ectopic pregnancies (high-risk pregnancies outside the uterus). It can also be responsible for prostate gland infections and infections near the testicles. However, chlamydia may be cured within one to two weeks with antibiotics.


Gonorrhea is an STI that can grow within the mouth, throat, eyes and anus. Symptoms usually appear within 14 days of exposure, but many men and most women with gonorrhea are asymptomatic.

Signs and symptoms may include:

  • Thick, cloudy or bloody discharge from the penis or vagina
  • Pain or burning sensation when urinating
  • Heavy menstrual bleeding or bleeding between periods
  • Painful, swollen testicles
  • Painful bowel movements
  • Anal itching, soreness, discharge or bleeding.

Although gonorrhea is considered to be curable with the right medication, antibiotic resistance is rendering this STI more difficult—sometimes even impossible—to treat. As a result, the CDC recommends that uncomplicated gonorrhea be treated only with the antibiotic ceftriaxone in combination with either azithromycin or doxycycline.

If untreated, gonorrhea can cause pelvic inflammatory disease in women. In men, it can cause a painful condition in the tubes attached to the testicles, which in rare cases can cause infertility. In addition, it can spread to a person’s blood or joints, a condition considered life-threatening.

Genital Herpes

Genital herpes is an STI caused by herpes simplex virus.

Individuals with herpes usually have few to no symptoms. The appearance of the first symptoms is known as an outbreak or first episode. Although these episodes can repeat for decades, some individuals never have a second one. Pain or itching may occur around the genitals, buttocks and inner thighs. Additionally, small red bumps, blisters or open sores that become ulcers can appear in the genital or anal areas. Eventually, the ulcers heal.

Unlike some other STIs, herpes cannot be cured. The virus remains dormant in the body until reactivated. Treatment can prevent or shorten outbreaks. If an individual does not receive adequate care, pain may worsen and can become severe in those with a suppressed immune system. Touching sores or fluids from sores may lead to the transfer of herpes to other parts of the body.

Human Papillomavirus

Human papillomavirus (HPV) is the most common STI in the United States. Currently, 79 million Americans have HPV, and 14 million new cases are diagnosed every year. At least 50 percent of sexually active individuals develop HPV at some point in their lives. There are over 100 different types of HPV, although some are riskier than others.

Many people are unaware of their HPV status because there are usually no signs or symptoms. HPV can increase the risk of genital warts and certain cancers, especially cervical cancer.

Signs and symptoms of genital warts may include:

  • Small, flesh-colored or gray swellings in the genital area
  • A cluster of warts that take on a cauliflower shape
  • Itching or discomfort around the genitals
  • Bleeding during intercourse.

Although HPV can clear up on its own, treatment may be needed. Vaccination is one way to prevent infection with the virus. It is recommended that all children ages 11 and 12 receive two doses of the HPV vaccine to protect against cancers caused by the virus. Catch-up vaccines are suggested for males through age 21 and females though age 26.


Syphilis is a curable bacterial infection transmitted via an infected sore during sexual activity. After the initial infection, it can lie dormant in the body for decades before it reactivates. The STI is divided into four stages: primary, secondary, latent and tertiary. Each stage is characterized by different signs and symptoms.

In the primary stage, a small sore called chancre appears at the site where the bacteria entered the body. Sometimes, multiple sores develop. Although the chancre usually appears three weeks after exposure, not all individuals with syphilis will notice it because it can be painless and hidden within the vagina or rectum. Within three to six weeks, the sore heals on its own regardless of treatment. But to prevent the infection from entering the secondary stage, treatment is required.

The secondary stage may include skin rashes or mucous membrane lesions. Rashes appear in one or multiple areas of the body and can appear while the primary sore is healing or after it’s healed. Other symptoms include hair loss, muscle aches, fever, sore throat and swollen lymph nodes.

During the latent stage of syphilis—which could last years—there are no visible signs or symptoms. Signs and symptoms may never return or the infection may enter the tertiary, or late-stage. About 15 to 30 percent of people with untreated syphilis will develop tertiary syphilis. Syphilis at this stage can lead to severe damage of the heart, brain and other organs and can be life-threatening. Treatment can help prevent future damage.


Trichomoniasis is an STI caused by a parasite called Trichomonas vaginalis. In the United States, an estimated 3.7 million people have the infection, and it is more common in women than men. Only 30 percent of individuals actually develop symptoms, which range from mild irritation to severe inflammation and appear within five to 28 days of exposure.

Symptoms include:

  • Clear, white, greenish or yellowish vaginal discharge
  • Discharge from the penis
  • Strong vaginal odor
  • Vaginal itching or irritation
  • Itching or irritation inside the penis
  • Pain during sexual intercourse
  • Painful urination.

Trichomoniasis is curable with oral medication. However, if untreated, it can last for months or years.

Although they’re not listed on this page, HIV, hepatitis and bacterial vaginosis (BV) are also common STIs.

How are STIs diagnosed?

Doctors diagnose certain STIs in different ways. Sometimes, a visual examination is sufficient to identify a particular infection. But some others require additional methods, such as blood, urine and Pap tests and swab samples.

How can you protect yourself against STIs?

The CDC recommends abstaining from sexual activity, using condoms, reducing the number of one’s sexual partners, getting vaccines and being in mutually monogamous relationships.

Getting tested regularly is one of the most important ways individuals can protect themselves from STIs. For more information on screening recommendations, click here.

Last Reviewed: February 3, 2020