Glossary · Dysbiosis and disease
Sexually transmitted infection (STI)
In one sentence
A sexually transmitted infection (STI) is an infection caused by bacteria, viruses or parasites that spreads mainly through vaginal, anal or oral sex. Many cause no symptoms, so testing is often the only way to find them.
Also known as: STI · STIs · sexually transmitted infections · STD · sexually transmitted disease · sexually transmitted diseases
What is a sexually transmitted infection?
STIs are infections passed on mainly through vaginal, anal or oral sex. Some can also spread through blood, or from mother to baby during pregnancy or birth. “Infection” has largely replaced “disease” (STD) because many people who carry an STI have no symptoms yet can still pass it on.
They are caused by different kinds of organism:
- Bacteria: Chlamydia trachomatis (chlamydia), Neisseria gonorrhoeae (gonorrhoea), Treponema pallidum (syphilis), Mycoplasma genitalium.
- Viruses: HIV, herpes simplex virus, human papillomavirus (HPV), hepatitis B.
- Parasites: Trichomonas vaginalis (trichomoniasis).
The WHO fact sheet reports that more than one million curable STIs are acquired every day worldwide among people aged 15–49, most of them without symptoms. It estimates around 374 million new infections in 2020 with one of four curable STIs: chlamydia, gonorrhoea, syphilis and trichomoniasis. Left undiagnosed, some are associated with infertility, ectopic pregnancy, pregnancy complications and a higher risk of acquiring HIV.
Not every genital complaint is an STI. Bacterial vaginosis and candidiasis are not classed as classic STIs, although their symptoms can overlap.
How STIs are detected
For many bacterial STIs the current standard is a nucleic acid amplification test, such as PCR, on a first-void urine sample or a swab. Syphilis and HIV are also tested by blood serology. Metagenomic sequencing can look for many organisms in one sample, including co-infections, and in some cases antimicrobial resistance genes, a growing concern with gonorrhoea.
Each infection has a window period, the time between exposure and the point at which a test can reliably pick it up. A negative result taken too early may need repeating.
Example
A GP sees a patient with a new partner after condomless sex, with no symptoms. She requests STI testing on first-void urine and blood. The report detects Chlamydia trachomatis and is negative for everything else. The GP arranges treatment, advises that recent partners are informed and tested, and books a follow-up test. The result is handled confidentially throughout.
STIs at Xenogene
Xenogene’s AMC STI (SeXitest) uses Nanopore sequencing to detect the microorganisms present in a sample. According to the product sheet, it covers bacteria including Chlamydia trachomatis, Neisseria gonorrhoeae, Treponema pallidum, Mycoplasma genitalium and Ureaplasma spp., as well as viruses such as herpes and HIV, and accepts first-void urine, urethral or vaginal swabs, and blood or serum. AMC Vaginal also includes detection of selected STIs alongside the microbiota profile. Tests are ordered through a healthcare professional, who is responsible for interpreting the result.
Frequently asked questions
What is the difference between an STI and an STD?
They refer to the same infections, but STI is now the preferred term. Disease implies symptoms, and many of these infections cause none while still being transmissible and able to cause complications.
How soon after sex can I get tested for STIs?
It depends on the infection and the test. Each has its own window period, and for some, such as HIV or syphilis, it can be several weeks. A sexual health clinician can advise which tests to do and when to repeat them.
Can you have an STI without symptoms?
Yes, and it is common. According to the WHO, most curable STIs are asymptomatic, which is why screening is recommended for people at risk even when they feel well.
Related terms
- PCR and qPCR
PCR (polymerase chain reaction) is a technique that copies a chosen stretch of DNA millions of times so it can be detected. qPCR, or real-time PCR, tracks amplification cycle by cycle, allowing the target to be quantified.
- Vaginal microbiota
The vaginal microbiota is the community of microorganisms living in the vagina. In many women of reproductive age it is dominated by Lactobacillus species, which produce lactic acid and help keep vaginal pH low.
- Bacterial vaginosis
Bacterial vaginosis is a shift in the vaginal microbiota in which lactobacilli decline and anaerobic bacteria such as Gardnerella overgrow. It can cause discharge and odour, but is often symptom-free.
- Candidiasis
Candidiasis is an infection caused by Candida yeasts, most often Candida albicans. It commonly affects the vulva and vagina or the mouth, and occurs when the yeast overgrows beyond normal colonisation.
- Antimicrobial resistance
Antimicrobial resistance (AMR) is the ability of bacteria, viruses, fungi or parasites to stop responding to medicines that used to control them, making infections harder to treat.
Educational content for healthcare professionals and the general public. It does not replace assessment by a healthcare professional: ordering and interpreting any test is the responsibility of the requesting clinician.
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