Common STIs: Symptoms, Testing and Prevention

Many people come to see me with the same question: "I don't have any symptoms at all. Do I really need a test?" The answer is yes. Many sexually transmitted infections are very quiet: no pain, no itching, no discharge, but they can still be passed on to the people we love, and some, if left for a long time, damage the body without us realising. This article gives you an easy-to-follow overview of which infections exist, how they're tested for and how well they can be prevented.

What are STIs, and how are they different from STDs?

STIs stands for sexually transmitted infections. Doctors now prefer this term to STDs (sexually transmitted diseases), because it emphasises that being infected doesn't always mean having symptoms or disease, yet you can still pass the infection on.

These infections spread through all kinds of sex: vaginal, anal and oral. Some, such as herpes and HPV, spread through direct skin-to-skin contact in the genital area. Condoms reduce the risk of these but don't prevent them completely.

Common infections by type of organism

Read more about each infection in syphilis, gonorrhoea, herpes and HIV basics.

It's worth knowing that in Thailand, syphilis has been rising steadily for several years among teenagers, working-age adults and pregnant women. Surveillance data from the Department of Disease Control for 2025 reported more cases than the previous year and above the five-year median. This is why I want blood testing to be a normal part of life for anyone who is sexually active.

STIs

Curable with medicine

Bacteria and protozoa

• Gonorrhoea (watch for resistance)
• Chlamydia
• Syphilis
• Trichomoniasis

Controllable viruses

• Herpes: symptoms well controlled
• HIV: treatment keeps it untransmittable

Preventable by vaccine

• HPV
• Hepatitis B

Curable with antivirals

• Hepatitis C
• Oral medicine can cure it

Many have no symptoms. No symptoms doesn't mean no infection.

Symptoms to watch for (but don't wait for symptoms)

• Burning or difficulty urinating, or pus or discharge from the urethra or vagina
• Unusual or smelly vaginal discharge, or unusual bleeding after sex
• Sores on the genitals, mouth or anus, whether painful or painless
• Clusters of small clear blisters with burning pain
• A rash on the body, palms or soles, or swollen lymph nodes
• A persistent sore throat after oral sex, or pain or bloody mucus from the anus

But let me stress: having no symptoms doesn't mean you're not infected. Most chlamydia in women causes no symptoms, gonorrhoea in the throat and rectum is usually silent, and latent syphilis can cause no symptoms for years.

Why you shouldn't leave it

If untreated, some infections spread upwards and cause pelvic inflammatory disease, blocked fallopian tubes, infertility or ectopic pregnancy. In men, they can cause inflammation of the testicles and the sperm ducts. Late-stage syphilis damages the brain, eyes and blood vessels, and some HPV types lead to cervical, anal and throat cancers.

Another very important point is that sores or inflammation from an STI make it easier to catch and pass on HIV. In pregnancy, syphilis can pass to the baby, which is why it's tested for at the first antenatal visit.

Who should be tested, and how often?

• Everyone who is sexually active should be tested for HIV, syphilis and hepatitis B at least once.
• People with a new partner or multiple partners should be tested before a new relationship and regularly afterwards.
• Men who have sex with men, trans women, people on PrEP and sex workers are advised to test every 3–6 months, including throat and rectal tests for gonorrhoea and chlamydia according to the type of sex they have, not just a urine test.
• Women who are sexually active should have cervical cancer screening according to their age.
• Couples planning to marry or have children can read more in premarital and couples STI testing.

PCR testing of urine or swabs can detect many infections at once; see the 14-pathogen PCR STI test. But some infections, such as syphilis and HIV, still require a blood test.

Everyone who has sex

• HIV, syphilis, hepatitis B at least once
• Women: cervical cancer screening by age

New or multiple partners

• Test before a new relationship
• Repeat periodically

Higher-risk groups

MSM, trans women, PrEP users, sex workers

• Test every 3-6 months
• Throat and anal swabs according to sexual practice
• Not just a urine test

Layered protection (combination prevention)

No single method protects against every infection, but combining several reduces risk a great deal.

1. Condoms with a water- or silicone-based lubricant, used correctly from start to finish. Read more in why lubricant matters more than you think.
2. Vaccines for HPV and hepatitis B; see the HPV vaccine for women and men.
3. PrEP to prevent HIV before exposure, and PEP after exposure within 72 hours. See the difference in PrEP vs PEP.
4. Doxy-PEP for some groups who keep getting syphilis or chlamydia; read doxy-PEP.
5. Regular testing, and treating partners at the same time, so infections don't pass back and forth.

When to see a doctor, and how the clinic can help

See a doctor soon if you have sores, discharge, a rash, or lower abdominal pain with fever, or if you've had a possible HIV exposure within the last 72 hours, which needs urgent assessment for PEP.

At Dr. Kris Clinic, I offer private risk assessments, blood tests and multi-pathogen PCR tests, interpretation of results and treatment plans, as well as PrEP/PEP care, at the clinic in Surat Thani and online across Thailand. Visitors from Koh Samui, Koh Phangan and Koh Tao are welcome, and English-speaking consultations are available. There's no need to feel embarrassed, and there is no judgement.