How is HIV transmitted?

Edwin J. Bernard
Published: 18 July 2010


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NB: Unless otherwise indicated, information in this chapter is summarised from NAM's publication HIV transmission & testing.

What scientists understand about HIV exposure or transmission is based on a combination of studying both the theoretical (what is biologically plausible) and the epidemiological (what has actually been seen in the real world). Assessing levels of HIV exposure or transmission risk requires using a commonsense perspective to balance these different types of evidence. For example, although it is known that saliva contains tiny amounts of HIV, and therefore transmission by this route is biologically plausible, there has never been a documented case of HIV infection resulting from kissing or spitting. Therefore, experts unanimously and emphatically agree that neither act imposes any risk of HIV transmission.

HIV is not present in:

  • faeces

  • sweat

  • urine.

HIV is sometimes present in negligible quantities in:

  • saliva

  • tears.

HIV is often – but not always – present in large enough quantities to infect in:

  • blood and blood products

  • rectal secretions

  • semen (and possibly pre-ejaculatory fluid)

  • vaginal and cervical fluids/secretions.

Biologically, four conditions need to be met for transmission to occur: HIV (as a whole virus or as a provirus – i.e., within cells that contain its genetic material) must be present in an infectious body fluid from the HIV-positive person; it must be present at sufficient levels to cause infection; there must be an effective route of transmission; and virus carried to another person via this route must reach susceptible cells in the other person.

HIV is transmitted in four main ways:

  • Via sex. Alleged sexual exposure or transmission are the focus of most HIV-related criminal prosecutions.

  • Via the use of non-sterile injecting equipment. The consensual use of non-sterile injecting equipment without disclosure of HIV-positive status is against the law in certain jurisdictions,i although few prosecutions have ever taken place. In very rare cases, individuals wielding needles/syringes that they claim contain HIV-infected blood have been arrested or prosecuted.1,2 There have also been a very small number of cases in which people have intentionally injected others with HIV.ii Additionally, medical workers, often working under poor infection-control conditions due to lack of resources, have sometimes been prosecuted for allegedly infecting others through the use of non-sterile injecting equipment.iii

  • From a mother to her infant. Prosecutions for this type of transmission, which can occur before or during birth, as well as during breastfeeding, have been extremely rare.3

  • Via medical use of blood products. HIV can be acquired via blood transfusion or the use of blood products if the blood has not been screened for HIV. Screening has taken place in wealthy countries since the mid-1980s, but 41 low- and middle-income countries are still not able to screen all blood donations.4 Some jurisdictions have laws against donating HIV-infected blood, but prosecutions are extremely rare.iv

The likelihood of HIV transmission occurring during sexual intercourse varies greatly from one situation to another. Factors that may increase or decrease the level of risk include:

  • the type of sexual activity, including whether or not intercourse is forced.

  • the roles during penetrative sex, in terms of who is the insertive partner and who is the receptive partner.

  • the amount of HIV in the bodily fluid to which the at-risk person is exposed.

  • the presence or absence of other sexually transmitted infections (STIs) in both partners.

  • for males, whether or not the penis has been circumcised.

  • protective factors in the potentially exposed partner, for example, the antiviral properties of saliva, and/or specific or non-specific immune defence mechanisms in genital secretions.

Importantly, most experts agree that the correct use of a male or female condom, regardless of other factors, subsantially reduces the risk of HIV transmission.5,6

i. For example, while Canada allows for prosecutions for HIV exposure via both the consensual use of non-sterile injecting equipment and consensual sex, but has only prosecuted the latter, Fiji only criminalises transmission via the consensual use of non-sterile injecting equipment: there are no laws against sexual transmission. See: No laws on HIV. Fiji Times, 21 April 2008.

ii. See, for example: ‘Father is guilty in HIV case’. New York Times 6 December 1998; Bernard EJ ‘Four Dutch men accused of “premeditated” criminal HIV transmission via rape and injection’., 1 June 2007; Eight years' prison for HIV infection. Sunday Star Times, 7 Feb 2010.

iii. For example, in 1999, six healthcare workers (five Bulgarian nurses and one Palestinian doctor) were accused of deliberately infecting children with HIV while working at a hospital in Benghazi, Libya, and received life sentences from a Libyan court; they were pardoned by the Bulgarian president upon their negotiated transfer to Bulgaria in 2007. Also in 2007, 21 Kazakh medical workers were found guilty of intentionally infecting children with HIV and were sentenced to up to five years in prison. See: BBC Online. ‘HIV medics released to Bulgaria’, 24 July 2007; Kaiser Network ‘Doctors in Kazakhstan on trial for medical malpractice following HIV outbreak among children who received blood transfusions., 20 March 2007; and Associated Press ‘21 convicted in Kazakh AIDS case’., June 27, 2007.

iv. In 2005, Singapore successfully prosecuted five blood donors whose blood tested HIV-positive but who were unaware they were living with HIV.7 The men had failed to declare their full sexual history. Prison sentences ranged between eight and ten months. A similar case – with a prison sentence of 33 months suspended for three years for a man unaware of his infection who failed to report a previous risk factor whilst donating blood – occurred in Thessaloniki, Greece in March 2010.


  1. Bernard EJ Canada: man who threatened mall cops with alleged HIV-infected needle gets eight months jail time., 15 May 2009
  2. Ruoff A Robber threatens restaurant employees with HIV infection., 2 February 2010
  3. GNP+ What price reproductive rights?, 2 December 2009
  4. WHO Global Database on Blood Safety., date accessed 30 July, 2010
  5. National Institute of Allergy and Infectious Diseases, National Institutes of Health, Department of Health and Human Services Scientific Evidence on Condom Effectiveness for Sexually Transmitted Disease (STD) Prevention. July, 2001
  6. Weller SC and Davis-Beaty K Condom effectiveness in reducing heterosexual HIV transmission. Cochrane Database of Systematic Reviews Issue 1, 2002
  7. Reuters Singapore jails five HIV blood donors for lying. 8 December, 2005

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A writer and advocate on a range of HIV-related issues, Edwin has a particular specialism in HIV and the criminal law. He works with national and international HIV organisations, including the International AIDS Society, GNP+ and UNAIDS, as well having as a long association with NAM as a writer on this topic and as the former editor of HIV Treatment Update. To visit Edwin's blog and respond to posts click here.

This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.
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This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.

NAM’s information is intended to support, rather than replace, consultation with a healthcare professional. Talk to your doctor or another member of your healthcare team for advice tailored to your situation.