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.
Feeling feverish, tired, or achy after sex is usually caused by physical exertion, dehydration, or a coincidental illness, not an STI. However, flu-like symptoms appearing 2-4 weeks after unprotected sex with a new partner can sometimes indicate acute HIV infection (seroconversion). Understanding the difference matters.
If you’re worried about symptoms you’re experiencing after sex, here’s how to assess the risk and decide whether you need to get tested.
Concerned? Get a confidential home STI test kit
Common Non-STI Causes of Flu Symptoms After Sex
Before assuming the worst, consider these much more common explanations:
- Physical exertion: Sex is exercise. Vigorous sex can cause muscle soreness, fatigue, and mild temperature elevation, especially if you’re not regularly active
- Dehydration: Can cause headache, fatigue, and lightheadedness
- Coincidental illness: You might simply have caught a cold or flu around the same time
- Anxiety and stress: Can cause physical symptoms including nausea, muscle tension, sweating, and elevated heart rate
- Urinary tract infection: Can cause fever, especially if it reaches the kidneys
- Allergic reaction: To latex condoms, lubricant, or spermicides. Can cause localised irritation and general malaise
When Flu Symptoms Could Be an STI
Acute HIV infection (seroconversion)
The one STI that genuinely causes flu-like symptoms is HIV. Around 60-80% of people experience seroconversion illness 2-4 weeks after contracting HIV. Symptoms include:
- Fever (often the first symptom)
- Severe fatigue
- Sore throat
- Body aches and joint pain
- Swollen lymph nodes (neck, armpits, groin)
- Rash (flat, reddish spots, usually on the trunk)
- Night sweats
- Mouth ulcers
These symptoms last 1-2 weeks and then resolve. They’re virtually identical to flu or glandular fever, which is why they’re almost never recognised as HIV at the time.
Key timing: If flu-like symptoms appear within 24-48 hours of sex, they’re almost certainly not HIV. Seroconversion symptoms take at least 2 weeks to appear (usually 2-4 weeks). If you feel unwell the day after sex, it’s physical exertion or coincidence.
Other STIs that can cause general malaise
- Primary herpes outbreak: A first genital herpes episode can cause fever, body aches, and swollen glands alongside the blisters. Timing: 2-12 days after exposure
- Hepatitis B: Can cause fatigue, nausea, loss of appetite, and jaundice. Timing: 6-12 weeks after exposure
- Syphilis (secondary): Can cause a low-grade fever, sore throat, and malaise alongside the rash. Timing: weeks to months after the initial chancre
Should You Get Tested?
Consider testing if:
- You had unprotected sex with a new partner and flu-like symptoms appeared 2-4 weeks later
- Symptoms include a combination of fever + rash + sore throat + swollen glands
- You know your partner is HIV-positive or from a high-prevalence population
- Your symptoms include genital blisters or sores alongside the flu-like symptoms
Testing windows: HIV from 4 weeks (conclusive at 90 days). Herpes: swab during outbreak. Syphilis: from 4-6 weeks. Hepatitis B: from 6 weeks.
Order a home test or visit your nearest sexual health clinic.
Read more: HIV symptoms in women | What is HIV? | STI symptom timelines
Frequently Asked Questions
Can you get a fever from an STI?
Yes, but it’s uncommon. Acute HIV, primary herpes, hepatitis B, and secondary syphilis can all cause fever. Chlamydia and gonorrhoea very rarely cause fever unless they’ve progressed to PID or epididymitis.
I felt sick right after sex. Is it an STI?
Almost certainly not. No STI causes symptoms within minutes or hours. Immediate symptoms are far more likely caused by physical exertion, anxiety, dehydration, or an allergic reaction.
Can anxiety after sex cause physical symptoms?
Absolutely. Anxiety about STIs can cause nausea, headache, muscle tension, rapid heartbeat, sweating, and digestive issues. These physical anxiety symptoms can feel very real and be mistaken for illness. If you’re anxious, getting tested provides the most effective relief.
Related reading
- Abnormal Genital Discharge and Chlamydia
- Asymptomatic STIs: Silent Infections Explained
- Causes of Painful Sex and How to Treat It
- Cold Sores vs Syphilis Sores: How to Tell the Difference
- Common STI Symptoms in Women
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.
