Sexually transmitted infections, commonly referred to as STIs, are conditions that are primarily passed from one person to another through sexual activity. While some are caused by bacteria, others result from viruses or parasites. Although STIs are common and treatable in many cases, they can lead to serious health problems when left undetected or untreated.

This article outlines various types of STIs, their symptoms, how they spread, preventive strategies, and when it’s necessary to see a healthcare provider.

What Are STIs?

STIs are infections transmitted mainly through sexual contact, including vaginal, anal, or oral sex. However, some can also spread through skin-to-skin contact, sharing personal items, or exposure to infected blood and bodily fluids.

Although symptoms may appear in some individuals, many STIs are asymptomatic, which means people may not realize they are infected and can unknowingly pass the infection to others.

Let’s explore some of the most common types of STIs.

Chlamydia

Caused by the bacterium Chlamydia trachomatis, chlamydia is a widespread infection that often goes unnoticed due to a lack of symptoms. It can be transmitted via oral, vaginal, or anal sex and can also be passed from mother to baby during childbirth.

When symptoms do occur, they might include:

  • Abnormal vaginal or penile discharge

  • Burning during urination

  • Rectal pain or bleeding in cases of anal infection

If left untreated, chlamydia can lead to complications such as pelvic inflammatory disease (PID) and infertility.

Pubic Lice (Crabs)

Pubic lice, also known as crabs, are tiny parasitic insects that typically infest hair in the genital area but may also be found in the armpits, chest, eyelashes, or beard.

Symptoms include:

  • Persistent itching

  • Visible small lice or eggs near hair follicles

These lice spread through close contact, especially during sexual activity, and sometimes via shared clothing or bedding. Over-the-counter creams and shampoos can treat most cases, but medical treatment may be needed if the lice are found on the eyelashes.

Herpes Simplex Virus (HSV)

Herpes is caused by two virus types: HSV-1 and HSV-2.

  • HSV-1 usually causes cold sores around the mouth but can also lead to genital infections through oral sex.

  • HSV-2 primarily causes genital herpes and spreads through vaginal, anal, or oral sex.

Symptoms may include:

  • Painful blisters or ulcers around the mouth or genitals

  • Burning while urinating

  • Unusual discharge

Once a person contracts HSV, the virus remains in the body and may reactivate periodically. While there’s no cure, antiviral medication can help manage outbreaks and reduce the risk of transmission.

Hepatitis B

Hepatitis B is a viral infection that affects the liver and can be transmitted through:

  • Sexual activity

  • Contact with infected blood or fluids

  • Sharing needles or other unsterile medical equipment

  • From mother to child during childbirth

Though many people recover on their own, chronic hepatitis B can lead to liver cirrhosis or cancer. Vaccination is the most effective prevention strategy.

Trichomoniasis

This infection is caused by a parasite called Trichomonas vaginalis. It's more common among women and can be passed through vaginal intercourse or vulva-to-vulva contact.

Symptoms may include:

  • Unusual vaginal or penile discharge

  • Pain during urination or sex

  • Genital itching

Some individuals may not experience any symptoms. Treatment involves prescription antibiotics, and both partners should be treated to prevent reinfection.

Human Immunodeficiency Virus (HIV)

HIV targets the immune system and can lead to acquired immunodeficiency syndrome (AIDS) if untreated. It spreads through:

  • Sexual contact

  • Sharing needles

  • Blood transfusions with infected blood

  • From mother to baby during childbirth or breastfeeding

Early stages may be symptom-free, but as the virus progresses, it compromises the immune system. Antiretroviral therapy (ART) helps manage the virus and reduce the viral load to undetectable levels, making it untransmittable when properly controlled.

Human Papillomavirus (HPV)

HPV is a group of viruses, some of which are linked to cancer, including cervical, anal, and throat cancer. It is one of the most common STIs globally.

Symptoms:

  • Genital warts

  • In many cases, no visible symptoms at all

HPV spreads through sexual contact and skin-to-skin contact. Vaccination, such as the Gardasil 9 vaccine, can prevent the most dangerous strains. Regular screening (e.g., Pap smears) is crucial for early detection of cervical changes.

Molluscum Contagiosum

Molluscum is a viral skin infection that causes small, round, dimpled bumps. Though not always classified as an STI, it can spread through skin contact during sex or sharing contaminated items.

In most cases, the condition resolves without treatment, but prescription medications or procedures like freezing or topical therapies can remove the bumps faster.

Scabies

Caused by mites that burrow under the skin, scabies leads to intense itching and a rash. It can spread through close contact, including sexual activity or sharing bedding or clothing.

Symptoms include:

  • Itchy rash, often worse at night

  • Small bumps or burrows on the skin

Topical creams are used for treatment, and all close contacts should be treated simultaneously to prevent reinfestation.

Syphilis

Syphilis progresses in stages:

  • Primary: A painless sore at the site of infection

  • Secondary: Rash, swollen lymph nodes, or flu-like symptoms

  • Latent: No symptoms, but the bacteria remain in the body

  • Tertiary: Severe complications affecting organs like the heart or brain

Syphilis spreads via sexual contact and from mother to baby. It’s easily treated with antibiotics in the early stages. Left untreated, it can cause irreversible damage.

Gonorrhea

Caused by Neisseria gonorrhoeae, gonorrhea can infect the genitals, rectum, throat, and eyes. It’s common and highly contagious through all forms of sex.

Symptoms may include:

  • Burning during urination

  • Discharge

  • Bleeding between periods

  • Rectal or throat pain

Antibiotic treatment is effective, though rising resistance to medications makes early detection important.

Chancroid

This bacterial infection is rare in many developed countries but still occurs elsewhere. It causes painful genital ulcers and may increase the risk of HIV transmission.

Treatment involves antibiotics, and recent sexual partners should also be examined.

How to Prevent STIs

While no method is 100% effective except abstinence, the following can greatly reduce STI risk:

  • Use condoms or dental dams consistently

  • Get tested regularly, especially with new or multiple partners

  • Limit the number of sexual partners

  • Avoid sharing needles

  • Get vaccinated for HPV and hepatitis B

  • Communicate openly with partners about STI history and testing

When to See a Doctor

If you experience any of the following, consult a healthcare provider:

  • Unusual discharge

  • Sores, rashes, or warts on the genitals

  • Pain during urination or sex

  • Flu-like symptoms following sexual activity

  • Known exposure to an STI

Even without symptoms, regular testing is essential—many STIs remain undetected for months or years. At-home STI tests are available but should be followed up with a healthcare professional for confirmation and treatment.

Summary

Sexually transmitted infections are widespread and affect people of all backgrounds. Conditions like chlamydia, herpes, HPV, and HIV can be managed or cured with early detection and appropriate treatment. Protecting yourself through regular testing, barrier methods, open communication, and vaccination can help reduce your risk.

Maintaining awareness and seeking prompt medical attention are key to managing STIs and preserving sexual health.