How transmission occurs

Published: 07 April 2009

HIV can be transmitted through – and, as far as essentially all evidence shows, only through – several well-established routes:

  • By sharing injecting equipment
  • By receiving blood transfusions or other blood-related products from an infected person
  • From an HIV-positive mother to her baby
  • Through risky sexual activities.

What constitutes a 'risky' activity (sexual or otherwise) is based on two kinds of data: theoretical (i.e., what is biologically plausible) and epidemiological (what has actually been seen in the 'real world').

Biologically, four conditions need to be present for transmission to occur: virus 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 it must reach susceptible cells in another person. These conditions will be discussed in more detail later, under The mechanisms of HIV infection. There are also physical and social co-factors (such as sexually transmitted infections and circumcision) which can substantially affect the risk of transmission.

Well-established, evidence-based guidelines have classified sexual activities into risk categories based on whether these biological conditions for transmission are present, and on the number of new infections that can be attributed to these behaviours. These risk categories are:

  • High risk: well-established, predominant modes of transmission with clear biological and epidemiological evidence.
  • Low risk: biologically plausible routes of transmission with relatively few attributable cases. The possible ambiguities and uncertainties inherent in this category are discussed in detail below.
  • Theoretical risk: biologically plausible routes with no clearly documented cases of transmission.
  • No risk: 'impossible' modes of transmission with no documented cases or biological plausibility.

The relative risks of specific sexual activities, including anal, vaginal and oral intercourse and other forms of sexual activity, are summarised in the next section and discussed at length in following chapters.

Comparative risks

Comparing the precise degrees of risk of specific sexual activities comes with various caveats:

  • People rarely engage in one sexual behaviour to the exclusion of all others.
  • Degrees of risk can vary significantly due to several co-factors.
  • Degrees of risk will vary according to the likelihood that a sexual partner has HIV infection.
  • Different studies have sometimes shown conflicting results in different populations.
  • In some cases, samples studies are small enough to allow statistical uncertainty.

Oral sex, for instance, is subject to several of the uncertainties just described. However, it is possible to describe risk according to categories or 'orders of magnitude'.

High-risk activities

  • Unprotected sexual intercourse: receptive or insertive, anal or vaginal.
  • Sharing unsterilised injecting equipment.
  • Being born to or breastfed by an untreated HIV-positive mother.
  • Receiving donated blood, organs or injections with unsterilised needles in countries with inadequate screening procedures.
  • Sharing uncleaned sex toys.

Less risky activities

  • Penetrative sex with appropriate barrier (condom or female condom), when used correctly: there is a (not insignificant) residual risk because incorrect use may lead the condom to fail.
  • Receptive oral sex with a man (giving a blow job). It is now thought that between less than 1% to 3% of all HIV cases in the UK may be due to oral transmission. Most cases suggest that oral transmission depends on either damaged tissue in the mouth or throat, the presence of an untreated sexually transmitted infection (STI) in the throat, or an ulceration on the penis. It may also need an exceptionally high viral load in the transmitting partner.
  • Sharing injecting equipment if properly sterilised.
  • Occupational risks in invasive surgical and medical contexts.
  • Laboratory work with superconcentrates of HIV.
  • Being born to or breastfed by an HIV-positive mother receiving antiretroviral treatment.
  • Any sexual activities where significant amounts of blood may be shared (e.g. fisting, piercing, etc). Small amounts of blood are unlikely to transmit HIV.
  • Blood in otherwise uncontaminated body fluids coming into contact with mucous membrane.

Theoretical risks

  • All forms of oral sex apart from performing fellatio.
  • Fingering and fisting.
  • Sharing cleaned penetrative sex toys.

No risk

Contact with body fluids on unbroken skin; kissing (no exchange of blood); coughing or sneezing; spitting; social contact, etc.

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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.