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HIV can be transmitted in three main ways:

Sexual transmission
Transmission through blood
Mother-to-child transmission
Wherever there is HIV, all three routes of transmission will take place. However the number of infections resulting from each route will vary greatly between countries and population groups.

To be successful, an HIV prevention programme must make use of all approaches known to be effective

For each route of transmission there are things that an individual can do to reduce or eliminate risk. There are also interventions that have been proven to work at the community, local and national level.

To be successful, an HIV prevention programme must make use of all approaches known to be effective, rather than just implementing one or a few select actions in isolation. The share of resources allocated to each area should reflect the nature of the local epidemic - for example, if most infections occur among men who have sex with men then this group should be a primary target for prevention efforts.

Although most of this page looks separately at each transmission route, it should be remembered that many people don’t fit into only one “risk category”. For example, injecting drug users need access to condoms and safer sex counselling as well as help to reduce the risk of transmission through blood.

First requirements

There are three key things that can be done to help prevent all forms of HIV transmission. First among these is promoting widespread awareness of HIV and how it can be spread. Media campaigns and education in schools are among the best ways to do this.

Another essential part of a prevention programme is HIV counselling and testing. People living with HIV are less likely to transmit the virus to others if they know they are infected and if they have received counselling about safer behaviour. In particular, a pregnant woman who has HIV will not be able to benefit from interventions to protect her child unless her infection is diagnosed. Those who discover they are uninfected can also benefit, by receiving counselling on how to remain that way.1 2

The third key factor is providing antiretroviral treatment. This treatment enables people living with HIV to enjoy longer, healthier lives, and as such it acts as an incentive for HIV testing. It also brings HIV-positive people into contact with health care workers who can deliver prevention messages and interventions. Studies suggest that HIV-positive people may be less likely to engage in risky behaviour if they are enrolled in treatment programmes. Nevertheless, it is also possible that widespread availability of treatment may make some members of the wider population less fearful of HIV infection, and hence less willing to take precautions.3 4

Sexual transmission

What works?

Someone can eliminate or reduce their risk of becoming infected with HIV during sex by choosing to:

Abstain from sex or delay first sex
Be faithful to one partner or have fewer partners
Condomise, which means using male or female condoms consistently and correctly
There are a number of effective ways to encourage people to adopt safer sexual behaviour, including media campaigns, social marketing, peer education and small group counselling. These activities should be carefully tailored to the needs and circumstances of the people they intend to help. Specific programmes should target key groups such as young people, women, men who have sex with men, injecting drug users and sex workers.5 6 7 Older people are also a group who require prevention measures, as an increasing number of new infections are occuring among those aged over 50.

Comprehensive sex education for young people is an essential part of HIV prevention. This should include training in life skills such as negotiating healthy sexual relationships, as well as accurate and explicit information about how to practise safer sex. Studies have shown that this kind of comprehensive sex education is more effective at preventing sexually transmitted infections than education that focuses solely on teaching abstinence until marriage.8 9

A condom vending machine in Vatican City

Numerous studies have shown that condoms, if used consistently and correctly, are highly effective at preventing HIV infection.10 Also there is no evidence that promoting condoms leads to increased sexual activity among young people. Therefore condoms should be made readily and consistently available to all those who need them.11

There is now very strong evidence that male circumcision reduces the risk of HIV transmission from woman to man by around 50%, which is enough to justify its promotion as an HIV prevention measure in some high-prevalence areas.12 However, studies suggest that circumcision does not reduce the likelihood of male-to-female transmission, and the effect on male-to-male transmission is unknown.13

Some sexually transmitted infections - most notably genital herpes - have been found to facilitate HIV transmission during sex. Treating these other infections may therefore contribute to HIV prevention.14 15 Trials in which HIV-negative people were given daily treatment to suppress genital herpes have found no reduction in the rate at which they become infected with HIV. Nevertheless, there is evidence to suggest that treating genital herpes in HIV positive people may reduce the risk of them transmitting HIV to their partners. Further research is ongoing.16

One group that shouldn’t be overlooked by HIV prevention programmes is those who are already living with the virus. Regular counselling can help HIV positive people to sustain safer sexual behaviour, and so avoid onward transmission.17 18

What are the obstacles?

It is usually not easy for people to sustain changes in sexual behaviour. In particular, young people often have difficulty remaining abstinent, and women in male-dominated societies are frequently unable to negotiate condom use, let alone abstinence. Many couples are compelled to have unprotected sex in order to have children. Others associate condoms with promiscuity or lack of trust.19

Some societies find it difficult to discuss sex openly, and some authorities restrict what subjects can be discussed in the classroom, or in public information campaigns, for moral or religious reasons. Particularly contentious issues include premarital sex, condom use and homosexuality, the last of which is illegal or taboo in much of the world. Marginalisation of groups at high risk - such as sex workers and men who have sex with men - can be a major hindrance to HIV prevention efforts; authorities are often unwilling to allocate adequate resources to programmes targeting these groups.

Safe male circumcision demands considerable medical resources and some cultures are strongly opposed to the procedure. has more about preventing sexual transmission of HIV.

Transmission through blood

What works?

People who share equipment to inject recreational drugs risk becoming infected with HIV from other drug users. Methadone maintenance and other drug treatment programmes are effective ways to help people eliminate this risk by giving up injected drugs altogether. However, there will always be some injecting drug users who are unwilling or unable to end their habit, and these people should be encouraged to minimise the risk of infection by not sharing equipment.20

Used syringes collected by a needle exchange in Puerto Rico
Needle exchange programmes have been shown to reduce the number of new HIV infections without encouraging drug use. These programmes distribute clean needles and safely dispose of used ones, and also offer related services such as referrals to drug treatment centres and HIV counselling and testing. Needle exchanges are a necessary part of HIV prevention in any community that contains injecting drug users.21

Also important for injecting drug users are community outreach, small group counselling and other activities that encourage safer behaviour and access to available prevention options.22

Transfusion of infected blood or blood products is the most efficient of all ways to transmit HIV. However, the chances of this happening can be greatly reduced by screening all blood supplies for the virus, and by heat-treating blood products where possible. In addition, because screening is not quite 100% accurate, it is sensible to place some restrictions on who is eligible to donate, provided that these are justified by epidemiological evidence, and don’t unnecessarily limit supply or fuel prejudice. Reducing the number of unnecessary transfusions also helps to minimise risk.23 24

The safety of medical procedures and other activities that involve contact with blood, such as tattooing and circumcision, can be improved by routinely sterilising equipment. An even better option is to dispose of equipment after each use, and this is highly recommended if at all possible.

Health care workers themselves run a risk of HIV infection through contact with infected blood. The most effective way for staff to limit this risk is to practise universal precautions, which means acting as though every patient is potentially infected. Universal precautions include washing hands and using protective barriers for direct contact with blood and other body fluids.25

What are the obstacles?

Despite the evidence that they do not encourage drug use, some authorities still refuse to support needle exchanges and other programmes to help injecting drug users. Restrictions on pharmacies selling syringes without prescriptions, and on possession of drug paraphernalia, can also hamper HIV prevention programmes by making it harder for drug users to avoid sharing equipment.