References UK clinical guidance from the NHS, the British HIV Association (BHIVA), and the British Association for Sexual Health and HIV (BASHH). Last updated: April 2026.
No, you cannot catch an STI from someone who doesn’t have one. But this is more complicated than it sounds because many people have STIs without knowing it — either because they have no symptoms, or because they haven’t tested. So the real question is usually: “Did my partner have an STI I didn’t know about?”
Here’s the honest answer.
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The Simple Answer
STIs are caused by specific bacteria, viruses or parasites. If someone doesn’t carry these organisms, they can’t pass them on. The complication is that many people with STIs don’t know they have them.
Why “Silent” Infections Are So Common
| STI | Percentage Who Have No Symptoms |
|---|---|
| Chlamydia | Up to 70-80% |
| Gonorrhoea (women) | Up to 50% |
| Gonorrhoea (men) | Up to 10% |
| Trichomoniasis | Up to 50% |
| HPV | Most people have no symptoms |
| Herpes | Many people have mild or no symptoms |
| HIV (early stage) | Often no specific symptoms for years |
| Hepatitis B and C | Often silent for years |
This is why someone can “not have an STI” in their own mind — simply because they have no symptoms — while actually being infected.
Other Reasons a Partner Might Not Know
- They’ve never been tested
- They tested negative within the window period (too early for reliable results)
- They were tested for some STIs but not others
- They acquired the infection after their last test
- They have a latent or dormant infection (like HPV or herpes)
When Partners Test Negative and You Still Get an STI
This can be genuinely confusing. Possible explanations:
- The partner’s test was too early after their own exposure
- They had an STI that wasn’t included in their test panel
- You have a previous infection that’s only now causing symptoms
- You acquired the STI from a different source
- False negative results (rare with modern testing)
The Importance of Comprehensive Testing
A good STI screen should include:
- Chlamydia and gonorrhoea (including throat and rectal swabs if relevant)
- HIV (4th-generation test)
- Syphilis
- Hepatitis B (and hepatitis C if indicated)
- Trichomoniasis if symptoms are present
Regular Testing Matters
If you’re sexually active, especially with new or multiple partners, regular testing catches silent infections. Recommended frequency:
- Sexually active under 25 — yearly
- New partner — after each new partner
- Multiple partners or MSM — every 3-6 months
- After any symptoms
Read more: When to test for STIs | How to avoid STIs | Home chlamydia tests
Frequently Asked Questions
If my partner tests negative, can I catch anything from them?
Only if they have a genuine infection that the test didn’t detect. Modern tests are highly accurate, but false negatives are possible in the window period.
Can I get HIV from someone who is undetectable?
No. U=U means that sustained undetectable viral load eliminates sexual transmission.
Should I blame my partner if I test positive?
Not necessarily. STIs can persist for a long time or come from earlier partners. Focus on treatment and open communication rather than blame.
Related reading
- A to Z of Sex: Sexual Health Terms Explained
- Alcohol and HIV: What You Need to Know
- Anxiety and HIV: Coping and Getting Support
- Are All STIs Curable?
- Blood Problems and HIV: Anaemia, Low Platelets and More
Sources
- NHS — Sexual health conditions
- BASHH — UK clinical guidelines for STI management
- British HIV Association (BHIVA) — HIV treatment guidelines
- Terrence Higgins Trust — sexual health information
- UK Health Security Agency — STI surveillance data
Steve Page is a recognised expert on Sexually Transmitted Diseases (STDs) and STD treatments, having published numerous articles in peer-reviewed journals and presented his research at conferences around the world. He has an in-depth understanding of the latest medical research on STDs, and is an advocate for the development of new treatments and protocols to improve the health of those affected. In addition to his research, he has dedicated his career to understanding the causes and symptoms of STDs, as well as how to best treat those impacted.
