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How HIV is transmitted

Sponges symbolising buttocks

For someone to get HIV, an infectious fluid like blood or semen has to get inside their body – usually during sex. This can happen if the person with HIV has a detectable viral load and no form of protection is being used.

How is HIV passed on during sex?

During sex body fluids from someone with HIV can get inside a person who is HIV negative.

If the person with HIV has a detectable viral load, the virus can enter the HIV negative person’s bloodstream. This can happen during vaginal and anal sex (and sometimes oral sex too, though this is much less common).

It can also happen when an object (eg, a sex toy) that has infectious body fluids on it is put inside an HIV negative person.

If someone with HIV is taking HIV medication and has an undetectable viral load they cannot pass on HIV.

Infection can be prevented by using a condom during sex, or by the HIV negative person taking pre-exposure prophylaxis (PrEP).

When is a person with HIV infectious?

Someone with HIV is infectious if they have a detectable viral load.

This is often during the first few months after infection when they have very high levels of the virus in their body fluids and may not yet have been diagnosed.

Early diagnosis means you can start treatment to protect your health and reduce your viral load to undetectable levels.

How HIV treatment stops HIV being passed on:

  1. A person with HIV who is taking treatment and has an undetectable viral load cannot pass on HIV.
  2. PrEP, when taken correctly, significantly reduces the chances of becoming HIV positive. PrEP is a course of HIV drugs taken by an HIV negative person to lower the chance of infection.
  3. Post-exposure prophylaxis (PEP), when started in time, can stop HIV infection after sex without a condom (or other exposure) with someone who is infectious - but it does not work every time. PEP is a month-long course of HIV medication taken by an HIV negative person after possible exposure to reduce the chance of getting HIV.
  4. If a woman is pregnant, HIV medication is part of the way mother to baby transmission can be prevented.

Can I get HIV without having sex?

Yes, HIV can also be passed on if you inject drugs and share injecting equipment (needles, syringes, swabs, spoons and other items) that has been used by someone with HIV.

Also, a woman can give birth to a baby who also becomes infected. This could happen during labour but can also take place while breastfeeding or in the womb before the baby is born.

This is now extremely rare in the UK because the following medical interventions can reduce the risk of mother-to-baby transmission to below 1 per cent:

  • the mother taking treatment if she is not already doing so
  • she may be offered a Caesarian birth if her viral load is high
  • the baby is given a course of antiretroviral treatment for the first few weeks
  • the mother not breastfeeding.

In countries that don’t have strict checks on the safety of their blood supply (this began in the UK in 1985), receiving contaminated blood can pass the virus on. This could also happen in countries that don’t screen other blood products, organs or sperm.

Read more about different modes of transmission:

  1. HIV transmission through semen and vaginal fluid ››
  2. infected blood ››
  3. mother to baby transmission ››
  4. ways HIV is NOT transmitted ››

Symptoms of HIV infection ›› ››



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  • In your reference to risks associated with sharing equipment, you state "Swabs, spoons ...." Although I understand the risk which could be present from sharing a hollow bore needle, just exactly how can sharing spoons be an issue ?

    Posted 08:33 Wed 10 Dec 2014

The Information Standard: Certified member

This article was last reviewed on 27/1/2017 by Anna Peters

Date due for the next review: 27/1/2020

Content Author: R. Scholey

Current Owner: Health promotion

More information:

Having a baby, Greta Hughson, NAM aidsmap, December 2015

No one with an undetectable viral load, gay or heterosexual, transmits HIV in the first two years of PARTNER study, Gus Cairns, NAM aidsmap, March 2014

Viral load and transmission, a factsheet for people with HIV, Gus Cairns, NAM aidsmap, September 2015

Viral load and transmission, a factsheet for HIV negative people, Gus Cairns, NAM aidsmap, September 2015

NAM, Transmission,  Aidsmap, (2012)

NAM, How Transmission Occurs, Aidsmap, (2012)

NAM, Viral Load in Semen

NAM, HIV in rectal secretions and role in HIV transmission

Christopher Gadd, Rectal Secretions From Men Who Have Sex With Men Contain More HIV Than Blood or Semen, NAM, Aidsmap, (2004)

Michael Carter, Plasma and Rectal Viral Load Correlated in HIV Positive Gay Men: Supports Use of Treatment as Prevention, (2011)

NAM, Estimated Risk Per Exposure, Aidsmap (2012)

NAM, Sexually Transmitted Infections, Aidsmap (2012)

Gus Cairns. No HIV transmissions from HIV-positive partner seen in Australian gay couples study, Aidsmap (2015).

Gus Cairns. No-one with an undetectable viral load, gay or heterosexual, transmits HIV in first two years of PARTNER study, Aidsmap (2014)

NHS Choices, HIV and AIDS 2014

Sexual Activity Without Condoms and Risk of HIV Transmission in Serodifferent Couples When the HIV-Positive Partner Is Using Suppressive Antiretroviral Therapy, Journal of the American Medical Association: Alison J. Rodger, MD; Valentina Cambiano, PhD; Tina Bruun, RN; Pietro Vernazza, MD; Simon Collins; Jan van Lunzen, PhD; Giulio Maria Corbelli; Vicente Estrada, MD; Anna Maria Geretti, MD; Apostolos Beloukas, PhD; David Asboe, FRCP; Pompeyo Viciana, MD1; Félix Gutiérrez, MD; Bonaventura Clotet, PhD; Christian Pradier, MD; Jan Gerstoft, MD; Rainer Weber, MD; Katarina Westling, MD; Gilles Wandeler, MD; Jan M. Prins, PhD; Armin Rieger, MD; Marcel Stoeckle, MD; Tim Kümmerle, PhD; Teresa Bini, MD; Adriana Ammassari, MD; Richard Gilson, MD; Ivanka Krznaric, PhD; Matti Ristola, PhD; Robert Zangerle, MD; Pia Handberg, RN; Antonio Antela, PhD; Sris Allan, FRCP; Andrew N. Phillips, PhD; Jens Lundgren, MD
JAMA. 2016;316(2):171-181. doi:10.1001/jama.2016.5148

Viral load, Michael Carter, Greta Hughson, NAM, Aidsmap, March 2014

More confidence on zero risk: still no transmissions seen from people with an undetectable viral load in PARTNER study, Gus Cairns, NAM, Aidsmap, July 2017

Open your eyes to STIs, NHS Choices, Nov 2015

When sex goes wrong, NHS Choices, Nov 2015

Pre exposure prophylaxis, Roger Pebody, NAM, Aidsmap, October 2016

Can post exposure prophylaxis (PEP) stop me getting HIV, NHS Choices, Sep 2015

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The easiest and most effective precaution to take against most STIs is using a condom.