Sexually Transmitted Infections: What Are They and When Should You Get Tested?

Sexually Transmitted Diseases (STDs), or more accurately Sexually Transmitted Infections (STIs), are infections transmitted primarily through sexual contact. They can be caused by bacteria, viruses, and parasites and can be transmitted through vaginal, anal, or oral sex. Some pathogens can also be transmitted through direct contact with skin or mucous membranes, through blood, and from mother to child during pregnancy or childbirth.

STIs are extremely common. According to the World Health Organization (WHO), more than one million new, curable sexually transmitted infections occur every day worldwide among people aged 15 to 49 years. Importantly, many of these infections cause no obvious symptoms. As a result, a person may be infected and transmit the pathogen without being aware of it.

Early diagnosis is important for both appropriate treatment and preventing transmission. If left untreated, certain STIs can lead to complications such as pelvic inflammatory disease, fertility problems, and complications during pregnancy, while specific infections are associated with an increased risk of certain cancers or of HIV transmission and acquisition.

What Are the Most Common Sexually Transmitted Infections?

More than 30 different bacteria, viruses, and parasites can be transmitted through sexual contact. The most common and clinically significant STIs include:

Bacterial and Parasitic Sexually Transmitted Infections
- Chlamydia: Chlamydia is caused by the bacterium Chlamydia trachomatis and is often asymptomatic. If left untreated, particularly in women, it can cause pelvic inflammatory disease and fertility problems.
- Gonorrhea: Gonorrhea is caused by Neisseria gonorrhoeae and can affect the urogenital tract, rectum, and pharynx. Many infections are asymptomatic, while increasing antibiotic resistance in N. gonorrhoeae is a significant public health concern.
- Syphilis: Syphilis is caused by Treponema pallidum and progresses through different stages, which may be separated by asymptomatic periods. If left untreated, it can lead to serious long-term complications.
- Trichomoniasis: Trichomoniasis is caused by the protozoan Trichomonas vaginalis and is often asymptomatic. It is one of the most common curable STIs worldwide.
- Mycoplasma genitalium infection: Mycoplasma genitalium can cause urethritis and cervicitis and has been associated with pelvic inflammatory disease. Its increasing resistance to antibiotics is of particular concern. Other mycoplasmas, such as Mycoplasma hominis, as well as Ureaplasma urealyticum/parvum, may be present in the urogenital tract without causing disease. Therefore, their detection is not automatically interpreted as a clinically significant STI.
Viral Sexually Transmitted Infections
- Human papillomavirus (HPV): HPV infection is extremely common and usually resolves spontaneously. Certain HPV types cause genital warts, while persistent infection with high-risk oncogenic types is associated with cervical cancer and other malignancies.
- Genital herpes: Genital herpes is caused by HSV-1 and HSV-2 and may cause recurrent blisters or ulcers. The infection may also be asymptomatic and can be transmitted even without visible lesions.
- HIV: HIV affects cells of the immune system and, if left untreated, can lead to AIDS. Modern antiretroviral therapy allows effective long-term management.
- Hepatitis B and C: These infections are transmitted primarily through blood, although sexual transmission is also possible, particularly for hepatitis B. Chronic infections can cause severe liver damage, while an effective vaccine is available against hepatitis B.

Symptoms and Signs of Sexually Transmitted Infections

Sexually transmitted infections can cause different manifestations depending on the pathogen, the site of infection, and sex. However, one of their most important characteristics is that many infections may remain asymptomatic, particularly in the early stages. For this reason, the absence of symptoms is not a reliable indication that an infection is not present. When symptoms occur, they may include:

unusual discharge from the vagina, penis, or anus
pain, burning, or discomfort during urination
itching, irritation, or redness in the genital area
blisters, ulcers, lumps, or other skin lesions on or around the genitals
pain in the lower abdomen or pelvic area
pain or swelling of the testicles
pain during sexual intercourse
unusual vaginal bleeding, for example between menstrual periods or after sexual intercourse
rectal symptoms, such as pain, discharge, or bleeding

Some STIs can also cause more general symptoms, such as fever, swollen lymph nodes, rash, or fatigue. At the same time, infections of the pharynx or rectum caused by pathogens such as Neisseria gonorrhoeae and Chlamydia trachomatis often do not cause characteristic symptoms.

The possibility of asymptomatic infection is also the main reason why preventive laboratory testing is particularly important. Testing can identify infections that would otherwise remain undiagnosed and could potentially be transmitted to others.

When Should You Get Tested for Sexually Transmitted Infections?

STI testing is not performed only when symptoms appear. Laboratory testing should be considered particularly in the following situations:

Preventively, at regular intervals: The frequency and type of testing should be individualized according to age, sexual history, and risk factors. More frequent testing may be recommended for people at increased risk.
After unprotected sexual contact or another potential exposure: The timing and type of testing depend on the pathogen and the nature of the exposure.
When possible symptoms of an STI occur: Such as discharge, painful urination, ulcers, blisters, or pelvic pain.
When a sexual partner has been diagnosed with an STI: Testing of sexual partners contributes to early diagnosis and helps limit further transmission.
When starting a new sexual relationship: Particularly before discontinuing condom use.
In people with new or multiple sexual partners or other factors associated with increased risk: More frequent preventive testing may be appropriate, even in the absence of symptoms.
During pregnancy or when planning a pregnancy: Testing for specific STIs is important for preventing mother-to-child transmission.

How Are Sexually Transmitted Infections Diagnosed?

The diagnosis of STIs is based on medical history, clinical evaluation, and appropriate laboratory testing. No single test can rule out all STIs, as the appropriate method depends on the pathogen, the type and anatomical site of potential exposure, the presence of symptoms, and the time elapsed since contact.

For several bacterial and parasitic infections, molecular methods such as PCR and other nucleic acid amplification tests (NAATs) detect the pathogen's genetic material. Depending on the suspected infection, urine, urethral, vaginal, or cervical samples may be tested and, when indicated, pharyngeal or rectal samples may also be required. For infections such as HIV, syphilis, and viral hepatitis, blood tests that detect antigens, antibodies, or a combination of both are primarily used.

How Soon After Potential Exposure Should You Get Tested?

STI tests cannot always provide a reliable result immediately after a potential exposure. Each infection and laboratory method has a different diagnostic window, meaning the period of time required before the relevant marker can be reliably detected.

For example, a laboratory-based fourth-generation HIV test using venous blood, which detects the p24 antigen and antibodies, can usually detect the infection approximately 18 to 45 days after exposure. Other tests and other STIs have different diagnostic windows.

Therefore, a negative result obtained too soon after a potential exposure does not always rule out infection, and repeat testing may be necessary in some cases. Choosing the appropriate sample is also important, as oral or anal sexual contact may require testing of a pharyngeal or rectal sample, respectively. Therefore, timing, test type, and appropriate samples should be determined based on the specific potential exposure.

Laboratory and Other Tests for the Investigation of STIs

Laboratory testing for STIs includes specific molecular and serological tests, depending on the microorganism being investigated. Individual tests or combined panels for multiple pathogens can be performed.

Sexually Transmitted Infections Screening (CheckUp A5): This is a combined molecular test that includes Chlamydia trachomatis, Neisseria gonorrhoeae, Mycoplasma genitalium, Mycoplasma hominis, Ureaplasma urealyticum/Ureaplasma parvum, and Trichomonas vaginalis. It allows the simultaneous detection of the genetic material of these microorganisms in an appropriate clinical sample.
Viral Sexually Transmitted Infections Screening (CheckUp A14): A combined blood test that includes fourth-generation HIV 1/2 testing, HBsAg for hepatitis B, and anti-HCV for hepatitis C. These tests are used to screen for important viral infections that can be sexually transmitted.
Syphilis Testing: Based on serological tests for antibodies against Treponema pallidum. The selection and interpretation of individual tests depend on the medical history, stage of infection, and any previous treatment.
Molecular HPV Testing: Detects the DNA of specific human papillomavirus types, with molecular testing for 41 low- and high-risk oncogenic HPV types. HPV testing is used primarily in the context of cervical cancer prevention rather than as general screening following every potential sexual exposure.

Depending on symptoms, medical history, and laboratory findings, additional examinations not part of basic STI testing may be required.

Gynecological examination: Evaluation of the genital organs when relevant symptoms or findings are present.
Pap test and colposcopy: Investigation of cellular abnormalities of the cervix.
Urological or andrological examination: Evaluation of persistent urogenital symptoms.
Pelvic or scrotal ultrasound: Used when complications are suspected.
Assessment of immune system function: In selected cases of persistent or recurrent infections, tests such as ImmuneScan® may be used as complementary tools to assess immune function, but they are not diagnostic tests for STIs.

Treatment and Prevention of Sexually Transmitted Infections

The treatment of STIs depends on the causative pathogen. Most bacterial and parasitic infections, such as chlamydia, gonorrhea, syphilis, and trichomoniasis, can be cured with appropriate medication. In contrast, for certain viral infections, such as HIV and genital herpes, treatment is primarily aimed at effectively controlling the infection and preventing complications.

Empirical use of antibiotics without prior medical evaluation should be avoided. Appropriate treatment depends on the specific pathogen, while antimicrobial resistance is a significant and growing concern, particularly in gonorrhea and Mycoplasma genitalium. Where indicated, testing and treatment of sexual partners are also important to reduce transmission and the risk of reinfection.

Prevention is based on the consistent and correct use of condoms, vaccination against HPV and hepatitis B, and informed and responsible communication between sexual partners. Preventive laboratory testing also plays an important role, as many sexually transmitted infections may not cause noticeable symptoms.

Conclusions: The Importance of Prevention and Regular Testing

The absence of symptoms does not rule out an STI. For this reason, laboratory testing should not be associated exclusively with the onset of symptoms or a known potential exposure, but can also form part of routine sexual health care.

The frequency and scope of preventive testing should be tailored to an individual’s sexual history and risk factors. Early diagnosis allows appropriate treatment, helps prevent complications, and reduces transmission, protecting both individual and public health.

References

World Health Organization. Sexually transmitted infections (STIs). Geneva: World Health Organization; 2025.
World Health Organization. Updated recommendations for the treatment of Neisseria gonorrhoeae, Chlamydia trachomatis, and Treponema pallidum (syphilis) and new recommendations on syphilis testing and partner services. Geneva: World Health Organization; 2024.
World Health Organization. Chlamydia. Geneva: World Health Organization; 2025.
World Health Organization. Syphilis. Geneva: World Health Organization; 2025.
World Health Organization. Herpes simplex virus. Geneva: World Health Organization; 2025.
European Centre for Disease Prevention and Control. Monitoring of the responses to sexually transmitted infection epidemics in EU/EEA countries, 2024. Stockholm: ECDC; 2025.
Harfouche M, AlMukdad S, Alareeki A, et al. Estimated global and regional incidence and prevalence of herpes simplex virus infections and genital ulcer disease in 2020: mathematical modelling analyses. Sex Transm Infect. 2025;101(4):214-223. Published 2025 May 19. doi:10.1136/sextrans-2024-056307.