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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 do you get HIV?

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

If someone with HIV is infectious they can pass on HIV through the following body fluids:

This can be prevented by using a condom during sex, or by the HIV negative person taking Pre-exposure prophylaxis (PrEP) or Post-exposure prophylaxis (PEP).

People who inject drugs can avoid HIV being passed on by not sharing drug injecting equipment.

During pregnancy your doctor will advise you how to protect your baby.

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 this allows the virus to 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.

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 reduce your viral load to undetectable levels, prevent transmission to others and protect your health.

What is protected sex?

Protected sex means using a male or female condom during sex if one partner has HIV and a detectable viral load.

Condoms should be used with water-based lubricant, as oil-based lube weakens their structure.

HIV treatment is also a form of protection.

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. It can take up to six months on treatment to become undetectable.
  2. Pre-exposure prophylaxis (PrEP) is a course of HIV drugs taken by an HIV negative person to lower the chance of infection. When taken correctly, PrEP significantly reduces the chances of becoming HIV positive. 
  3. Post-exposure prophylaxis (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. When started in time, PEP can stop HIV infection after sex without a condom (or other exposure) with someone who is infectious - but it does not work every time. 
  4. If a woman is pregnant, HIV medication is part of the way mother to baby transmission can be prevented.

The evidence in study results

A large study called PARTNER looked at 888 gay and straight couples (and 58,000 sex acts) where one partner was HIV positive and one was HIV negative.

Results showed that where the HIV positive partner was on effective treatment and had an undetectable viral load, there were no cases of HIV transmission whether they had anal or vaginal sex without a condom.

The Partners PrEP study looked at the risk of transmission in the time it took the HIV positive partner to achieve an undetectable viral load. In most cases (84.8%) this happened within six months and there were no infections once someone had been on treatment for six months or more.

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.

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: 26/1/2020

Content Author: R. Scholey

Current Owner: Health promotion

More information:

HIV Transmission Risk Persists During the First 6 Months of Antiretroviral Therapy, Mujugira A1, Celum C, Coombs RW, Campbell JD, Ndase P, Ronald A, Were E, Bukusi EA, Mugo N, Kiarie J, Baeten JM; Partners PrEP Study Team
National Center for Biotechnology Information
US National Library of Medicine
2016 Aug 15;72(5):579-84. doi: 10.1097/QAI.0000000000001019

HIV treatment as prevention and HPTN 052, Cohen MS1, McCauley M, Gamble TR
National Center for Boiotechnology Information
US National Library of Medicine

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

Can't pass it on

People on effective treatment can't pass on HIV

If everyone knew this, we could bring an end to stigma and stop HIV transmissions.

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