The global HIV pandemic

The human immunodeficiency virus (HIV), first discovered in 1984, attacks the body’s immune system. If left undiagnosed and/or untreated, HIV can gradually weaken the immune system, making it more difficult for the infected person to fight off disease.

Acquired Immune Deficiency Syndrome (AIDS), first recognised in 1981, is the name for a collection of symptoms and illnesses that can occur if an HIV-positive person’s immune system becomes very weak as a result of HIV-related underlying immune deficiency. Since AIDS is not a disease, and because HIV infection does not inevitably lead to any one of these symptoms or illnesses, the term 'HIV-related illness' is now considered more accurate.

HIV is usually acquired in one of three ways:

• through unprotected anal or vaginal sex.
• from a mother to her baby.
• through blood-to-blood contact, including sharing non-sterile drug-use equipment.

HIV infection most commonly occurs as a result of blood, semen, vaginal fluid, anal secretions or breast milk from a person with HIV entering another person’s body.

HIV exposure occurs when there is an opportunity for HIV to enter a person’s body, for example, through unprotected anal or vaginal sex. However, HIV is not easy to acquire. Whilst it is possible to be sexually exposed to HIV just once and become infected, it is very much more likely that this will be not the case. Some people are sexually exposed to HIV many hundreds of times and never become infected. Consequently, although most cases of HIV infection worldwide are attributable to sexual exposure, an act of sexual intercourse with an HIV-positive person is more likely than not to leave an HIV-negative person uninfected. Read more in the chapter: Risk.

Global HIV incidence – the number of new HIV infections per year – is now estimated to have peaked in 1996 with 3.5 million new infections per year.¹ In 2008, the estimated number of new HIV infections was approximately 30% lower than at the epidemic’s peak 12 years earlier.¹ The number of new infections appears to have stabilised in most regions of the world, although it is still increasing in Eastern Europe and Central Asia and in other parts of Asia.¹

Worldwide, half of the estimated 33.4 million people living with HIV in 2008 were female¹ and HIV is the leading cause of disease and death for women of childbearing age.² Sub-Saharan Africa remains the global region most affected by HIV and AIDS – more than two-thirds of all people with HIV live in sub-Saharan Africa. In high-income countries with a low overall prevalence of HIV, sub-populations most affected by HIV include: gay men and other men who have sex with men; migrants; people who inject drugs; and sex workers.¹

Researchers first succeeded in developing effective anti-HIV drugs in the mid-late 1990s. Combinations of these drugs, which are known collectively as antiretroviral therapy (ART), reduce levels of HIV in the blood and other bodily fluids, allowing the immune system to recover, and reducing the possibility of infectiousness. Most people who take ART on an ongoing basis are able to maintain good health for many years. ART is the standard of care for HIV treatment in wealthy countries, and is gradually becoming the standard of care in resource-limited countries. Read more in the chapter: Harm

The global number of people living with HIV is constantly increasing not only because people are continuing to acquire HIV, but also because effective treatment has resulted in significantly extended lifespans for people who have had timely access to HIV testing and counselling, and can access treatment and care.^3,4,5,6,7

However, for every two people starting treatment another five are newly infected.¹ Continued high rates of transmission of HIV largely result from a failure to use currently available, evidence-informed, effective prevention strategies and tools, and poor coverage of HIV-prevention programmes.⁸ Globally, less than one person in five at risk of acquiring HIV has access to basic HIV-prevention services.⁹

Three decades of experience have led to many evidence-informed HIV-prevention activities. These fall into the following broad categories.

These include: male and female condoms; sterile drug-use equipment; male circumcision; and the use of ART to prevent infection in people exposed to HIV, known as post-exposure prophylaxis (PEP). Biomedical approaches currently being investigated include microbicides and novel uses of ART. Microbicides work like ‘invisible condoms’ creating a barrier to sexual transmission but, unlike condoms, are controlled by the receptive partner. ART may also be effective when taken by people at high risk of acquiring HIV. This is known as pre-exposure prophylaxis (PrEP).

ART's preventive effects are also being studied in populations that include people who are diagnosed with HIV and are taking treatment primarily for its health benefits. This is known as 'treatment as prevention'. Read more in the chapter: Risk

These include voluntary counselling and testing for HIV antibodies (the 'HIV test'), and counselling to support people living with, and at risk of, HIV to help them reduce HIV-related risk behaviours. (See Supporting people living with HIV to protect themselves and others.)

These include sex education and HIV-related information in schools, as well as 'safer sex' and HIV-testing campaigns, and HIV-related information targeted at specific communities where HIV prevalence is higher than in the general population.

These include legal and policy interventions that may permit some of the other types of prevention work, such as condom distribution, provision of sterile drug-use equipment, or sex education in schools. Examples of structural interventions include reforming laws and policies, and strengthening their enforcement, in order to better address the drivers of vulnerability to HIV acquisition, such as gender inequality, violence, discrimination, economic inequality, and lack of social capital. (See also The role of the law in the global response to HIV.)