HIV testing and risks of sexual transmission

This guide to HIV testing and HIV transmission was updated in June 2021.

The main change since the previous edition is that UK guidelines (2020) now recommend waiting six weeks before having an HIV test. See: What is the window period for an HIV test?

There is also a stronger emphasis throughout the guide on U=U and PrEP.

  • U=U: how an undetectable viral load on ART stops HIV transmission.
  • PrEP: how effectively a pill called PrEP prevents transmission. PrEP is now available free in the UK on the NHS.

This guide was produced to answer all questions about HIV testing. It also answers all questions on the risks of transmission.


HIV in the UK

HIV basics

Ways that HIV is not transmitted

Risks for transmission

HIV testing: feelings of fear, anxiety and guilt

Frequently Asked Questions (FAQs)

PrEP (Pre-Exposure Prophylaxis)


U=U: Undetectable = Untransmittable

HIV testing

Skin, mucous membranes and HIV transmission

Timeline for immune responses and testing

Additional sections


Further information and links




Tables, diagrams and illustrations


This guide was written and compiled by Simon Collins for HIV i-Base with appendices written by Charlotte Walker.

1st edition February 2012.
2nd edition February 2013.
3rd edition July 2016.
4th edition, December 2019.
5th edition, June 2021.

Thanks to the community and healthcare professional advisors for comments including Dr David Asboe, Bisi Alimi, Polly Clayden, Paul Clift, Nathan Geffen, Li Marhaban, Angelina Namiba, Emma Rezel, Memory Sachikonye, Tracy Swan, Matt Williams.

Thanks to MAC AIDS Fund for supporting this publication.

About our guides

Information about how we produced this guide and the importance of using language that is direct and easy to understand.

This includes information on how to write non technical medical information that may be useful as a resource for other organisations.

Cover: Untitled, 1984 © Keith Haring Foundation. Used with permission.

Last updated: 1 June 2021.