Sexually transmitted infections (STIs)

Key facts

• More than 1 million curable sexually transmitted infections (STIs) are acquired every day worldwide in people 15–49 years oldthe majority of which are asymptomatic.
• In 2020 there were an estimated 374 million new infections in people 15–49 years with 1 of 4 curable STIs: chlamydiagonorrhoeasyphilis and trichomoniasis.
• An estimated 8 million adults between 15 and 49 years old were infected with syphilis in 2022.
• An estimated 520 million people aged 15–49 years (13%) worldwide have herpes simplex virus type 2 (HSV-2) infectionthe main cause of genital herpes (1).
• Human papillomavirus (HPV) infection is associated with over 311 000 cervical cancer deaths each year (2).
• 1.1 million pregnant women were estimated to be infected with syphilis in 2022resulting in over 390 000 adverse birth outcomes.
• STIs have a direct impact on sexual and reproductive health through stigmatizationinfertilitycancers and pregnancy complications and can increase the risk of HIV.
• Drug resistance is a major threat to reducing the burden of gonorrhoea worldwide.

Overview

More than 30 different bacteriaviruses and parasites are known to be transmitted through sexual contactincluding vaginalanal and oral sex. Some STIs can also be transmitted from mother-to-child during pregnancychildbirth and breastfeedingand unsafe blood transfusion. Eight pathogens are linked to the greatest incidence of STIs. Of these4 are currently curable: syphilisgonorrhoeachlamydia and trichomoniasis. The other 4 are viral infections: hepatitis Bherpes simplex virus (HSV)HIV and human papillomavirus (HPV).

In additionemerging outbreaks of new infections that can be acquired by sexual contact such as mpoxShigella sonneiNeisseria meningitidisEbola and Zikaas well as re-emergence of neglected STIs such as lymphogranuloma venereum. These herald increasing challenges in the provision of adequate services for STIs prevention and control.

Scope of the problem

STIs have a profound impact on sexual and reproductive health worldwide.

More than 1 million curable STIs are acquired every day. In 2020WHO estimated 374 million new infections with 1 of 4 STIs: chlamydia (129 million)gonorrhoea (82 million)syphilis (7.1 million) and trichomoniasis (156 million). More than 520 million people were estimated to be living with genital herpes in 2020and an estimated 300 million women have an HPV infectionthe primary cause of cervical cancer and anal cancer among men who have sex with men. In additionupdated WHO estimates that 254 million people were living with chronic hepatitis B in 2022.

STIs can have serious consequences beyond the immediate impact of the infection itself.

• STIs like herpesgonorrhoea and syphilis can increase the risk of HIV acquisition.
• Mother-to-child transmission of STIs can result in stillbirthneonatal deathlow-birth weight and prematuritysepsisneonatal conjunctivitis and congenital deformities.
• HPV infection causes cervical and other cancers.
• In 2022hepatitis B resulted in an estimated 1.1 million deathsmostly from cirrhosis and hepatocellular carcinoma (primary liver cancer).
• STIs such as gonorrhoea and chlamydia are major causes of pelvic inflammatory disease and infertility in women.

Prevention of STIs

When used correctly and consistentlycondoms offer one of the most effective methods of protection against STIsincluding HIV. Although highly effectivecondoms do not offer protection for STIs that cause extra-genital ulcers (i.e.syphilis or genital herpes). When possiblecondoms should be used in all vaginaloral and anal sex.

Safe and highly effective vaccines are available for 2 viral STIs: hepatitis B and HPV. These vaccines have represented major advances in STI prevention. To eliminate cervical cancer as a public health problem globallyhigh coverage targets for HPV vaccinationscreening and treatment of precancerous lesionsand management of cancer must be reached by 2030 and maintained at this high level for decades.

The development of vaccines against STIs are a major priority to control STIs worldwide. Currently there is mounting evidence suggesting that the vaccine to prevent meningitis (MenB) provides some cross-protection against gonorrhoea. WHO is closely monitoring the results of ongoing studies to gather evidence. Vaccine candidates against herpes simplex viruschlamydia and gonorrhoea are in early clinical developmentwhile those against syphilis and trichomoniasis are still in the pre-clinical phase.

WHO is currently working on recommendations for the use of doxycycline as post-exposure prophylaxis (also known as Doxy-PEP) for selected bacterial STIs for some populations at higher risk of infection. Other biomedical interventions to prevent some STIs include adult voluntary medical male circumcisionmicrobicidesand partner treatment. 

Diagnosis of STIs

STIs are often asymptomatic. When symptoms occurthey can be non-specific.

Accurate diagnostic tests for STIs (using molecular technology) are widely used in high-income countries. These are especially useful for the diagnosis of asymptomatic infections. Howeverthey are largely unavailable in low- and middle-income countries (LMICs) for chlamydia and gonorrhoea. Even in countries where testing is availableit is often expensive and not widely accessible. In additionthe time it takes for results to be received is often long. As a resultfollow-up can be impeded and care or treatment can be incomplete.

On the other handinexpensiverapid tests are available for syphilishepatitis B and HIV. The rapid syphilis test and rapid dual HIV/syphilis tests are used in many resource-limited settings.

Several other rapid tests are under development and have the potential to improve STI diagnosis and treatmentespecially in resource-limited settings.

Screening of asymptomatic STIs is recommended in selected priority populations and settings.

Treatment of STIs

Effective treatment is currently available for several STIs.

• Three bacterial (chlamydiagonorrhoea and syphilis) and one parasitic STIs (trichomoniasis) are generally curable with existing single-dose regimens of antibiotics.
• For herpes and HIVthe most effective medications available are antivirals that can modulate the course of the diseasethough they cannot cure the disease.
• For hepatitis Bantivirals can help fighting the virus and slowing damage to the liver.

AMR of STIs – in particular gonorrhoea – has increased rapidly in recent years and has reduced treatment options. The Gonococcal AMR Surveillance Programme (GASP) has shown high rates of resistance to many antibiotics including quinoloneazithromycin and extended-spectrum cephalosporinsa last-line treatment (3).

AMR for other STIslike Mycoplasma genitaliumalso exist but are not systematically monitored.

STI case management

LMICs rely on identifying consistenteasily recognizable signs and symptoms to guide treatmentwithout the use of laboratory tests. This approach – syndromic management – often relies on clinical algorithms and allows health workers to diagnose a specific infection based on observed syndromes (e.g.vaginal/urethral dischargeanogenital ulcersetc). Syndromic management is simpleassures rapidsame-day treatmentand avoids expensive or unavailable diagnostic tests for patients with symptoms. Howeverthis approach results in overtreatment and missed treatment as the majority of STIs are asymptomatic. ThusWHO recommends countries to enhance syndromic management by gradually incorporating laboratory testing to support diagnosis. In settings where quality assured molecular assays are available, it is recommended to treat STIs such as gonorrhoeachlamydiasyphilisTrichomonas vaginalisMycoplasma genitaliumCandida albicansbacterial vaginosis and human papillomavirus (anogenital warts)based on laboratory tests and with the most updated treatment evidence-based regimens. MoreoverSTI screening strategies are essential for those at higher risk of infectionsuch sex workersmen who have sex with menadolescents in some high-burden settings and pregnant women.

To interrupt transmission and prevent re-infection, treating sexual partners is an important component of STI case management.

WHO response

Our work is currently guided by the Global health sector strategy on HIVHepatitis and Sexually Transmitted Infections2022–2030. Within this frameworkWHO:

• develops global targetsnorms and standards for STI preventiontesting and treatment;
• supports the estimation and economic burden of STIs and the strengthening of STI surveillance;
• globally monitors AMR to gonorrhoea; and
• leads the setting of the global research agenda on STIsincluding the development of diagnostic testsvaccines and additional drugs for gonorrhoea and syphilis.

As part of its missionWHO supports countries to:

• develop national strategic plans and guidelines;
• scale-up primary prevention (condom availability and useetc.);
• increase integration of STI services within primary healthcare services;
• increase accessibility of people-centred quality STI care;
• facilitate adoption of point-of-care tests;
• enhance and scale-up health intervention for impactsuch as hepatitis B and HPV vaccinationsyphilis screening in priority populations;
• strengthen capacity to monitoring STIs trends; and
• monitor and respond to AMR in gonorrhoea.
  
  

References

1. Harfouche MAlMukdad SAlareeki AOsman AMMGottlieb SRowley JAbu-Raddad LJLooker KJ. 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 May 19;101(4):214-223. doi: 10.1136/sextrans-2024-056307. PMID: 39658199
2. Bray FFerlay JSoerjomataram ISiegel RLTorre LAJemal A. Global cancer statistics 2018: GLOBOCAN estimates of incidence and mortality worldwide for 36 cancers in 185 countries. CA Cancer J Clin. 2018 Nov;68(6):394-424. Epub 2018 Sep 12. Erratum in: CA Cancer J Clin. 2020 Jul;70(4):313.
3. Unemo MLahra MMEscher MEremin SCole MJGalarza PNdowa FMartin IDillon JRGalas MRamon-Pardo PWeinstock HWi T. WHO global antimicrobial resistance surveillance (GASP/GLASS) for Neisseria gonorrhoeae 2017-2018: a retrospective observational study. Lancet Microbe 2021; 2: e627–36