(July 2002) The HIV/AIDS epidemic is a crisis of enormous proportions that is rapidly wiping out many of the gains sub-Saharan Africa has achieved since the countries attained independence. Bringing the epidemic under control is now perhaps the foremost development imperative for this part of the world.

A Few Basic Terms

The UNAIDS-recommended measure to understand the extent of HIV in a population is the HIV prevalence among 15 to 49 year olds, or the percentage of people in that age group who are infected with the virus. Despite the epidemic’s impact and high profile, the measures used to describe it are often used inaccurately. For example, when Zimbabwe is described as having 25 percent HIV prevalence, it means that 25 percent of 15 to 49 year olds in the country are thought to have HIV; it does not mean that 25 percent of all Zimbabweans or one-quarter of the Zimbabwean population has the virus.

Adult HIV incidence is the percentage of persons 15 to 49 years old who become newly infected each year. It is almost impossible to collect consistent incidence data for most of the world’s countries, but the concept is widely used in computer simulation modeling and in UNAIDS global reporting and is important for understanding the epidemic.

What Are HIV and AIDS?

The human immunodeficiency virus (HIV) is the virus that causes acquired immune deficiency syndrome (AIDS). Once the virus is in the human body, it multiplies and acts by weakening the immune system. The immune system produces special cells called antibodies to stave off microorganisms that might infect the body. With a weakened immune system, however, the body is highly susceptible to infections and is less able to fight off disease.

When the immune system becomes seriously compromised, the illness progresses to AIDS. AIDS, therefore, is defined by the degree of deterioration of the immune system, which in turn is defined by the extent of opportunistic infections that take advantage of the weakened immune system. Nearly all Africans who have HIV eventually die from AIDS-related illnesses, most within 10 years of infection.1 In most of the developing world, tuberculosis is the most common opportunistic infection for people living with HIV/AIDS.2


HIV is a lentivirus, meaning that it is slow acting in its pathogenic impact on the human body. To date, scientists have identified two types of HIV, namely HIV-1 and HIV-2. Both are transmitted in the same manner and result in clinically similar AIDS. The difference is that HIV-2 appears to be less easily transmitted than HIV-1, and the period between infection and full-blown AIDS is longer with HIV-2. HIV-1 predominates both worldwide and in Africa. Although HIV-1 is the dominant strain in West Africa as well, this part of the continent has relatively more HIV-2 than elsewhere.3 This may be one contributing factor to the slower development of the epidemic in West Africa.

HIV-1 is an extremely variable virus that comprises several different strains that can be classified within two groups. Group M is the major group and includes at least 10 subtypes of viruses. The currently recognized ones are subtypes A to J. Group O (outliers) is a catch-all category that includes a variety of relatively uncommon but distinct viruses. Group M subtypes are unevenly distributed throughout the world. In the sub-Saharan region, subtypes A and D predominate. Subtype C is common in South Africa (and India), while subtype E dominates in the Central African Republic. While subtype B is the dominant strain in the industrialized world, it is much less prevalent in Africa.4

The major difference among subtypes is in their genetic composition. Scientists have postulated that certain subtypes may be more associated with particular modes of transmission (homosexual versus heterosexual, for example) than others, or that certain subtypes might be more easily transmitted than others. The scientific evidence is inconclusive.

Similarly, no one quite knows what the implications of the different subtypes are for vaccine research. On the one hand, the predominant subtypes found in Africa are not the same as those found in industrialized countries; on the other hand, most of the funding for vaccine research comes from countries outside of Africa. Critics wonder if a vaccine designed to address developed country subtypes — even if available in the distant future — will prove efficacious in Africa. (Even worse is the possibility that the ability of HIV to mutate and adapt will make the quest for a vaccine forever futile.) On the other side, it may also turn out that a single vaccine is suitable for all subtypes. In addition, critics warn that it is simply impractical to think of developing a vaccine for each major subtype.

Transmission Mechanisms

Once in a population, how is HIV transmitted from person to person? An infected person can transmit HIV through sexual contact (heterosexual or homosexual) with uninfected partners. An infected woman can also transmit the virus to her infant during pregnancy, delivery, or while breastfeeding. A person can also become infected through transfusion of contaminated blood or by sharing needles used for injections and drug use. An important characteristic of the HIV/AIDS epidemic in sub-Saharan Africa is that heterosexual contact is identified as the main mode of transmission.5 Mother-to-child transmission is, of course, very important, but in the case of sub-Saharan Africa, mothers become infected in the first place through heterosexual contact.

Incubation Period

One of the major problems that Africa has had in coming to terms with HIV/AIDS is that in many ways, it has been a hidden epidemic. An HIV-infected adult6 does not develop AIDS immediately. Rather, a lengthy period from HIV infection to development of the clinical disease exists that may last for two to 12 years or even longer. (Some people may develop AIDS symptoms more quickly, while others may go beyond 12 years without developing full-blown AIDS.)7

An individual can transmit HIV to others as soon as he or she is infected. For most of the incubation period, the infected person may or may not know that he or she is infected. This contributes to the spread of HIV/AIDS because the person can unknowingly transmit the infection to others.

An HIV/AIDS pyramid with a small peak and a much larger body below gives a sense of the hidden nature of this epidemic. Because of the long and variable period before an infected person develops AIDS, and because most people die quite quickly after they develop full-blown AIDS, actual AIDS cases at any given time are just the tip of the epidemic. A great many more people are HIV-infected but have not yet developed AIDS. The great majority of these do not even know their status.8

Sentinel Surveillance

If the epidemic is hidden in this manner, then how do African officials learn about levels of infection and trends? Sentinel surveillance systems are the most common source of information. In a given year for a given period, a country designates certain health centers or hospitals as sentinel surveillance sites. At these selected sites, health workers have to take blood samples from certain groups of patients in the normal course of treatment. For reporting purposes, pregnant women (antenatal care clients) visiting for the first time for the current pregnancy are the most important of these groups because they serve as representative of the general population.9 The blood samples are unlinked to the patients — the testers cannot identify the source of the blood sample — and are then tested anonymously to provide information on the status and course of the epidemic.

In selected regions of a limited number of countries, population-based surveys have also been organized to obtain information on the HIV/AIDS epidemic. These differ from the sentinel surveillance surveys in that samples are taken from both men and women and for all age groups. While these provide invaluable information on trends, risk factors, and behavior change, they are much more expensive and technically difficult to organize than the sentinel surveillance. DHS has done national surveys on HIV/AIDS. In areas that have had both population-based and sentinel surveys, the two can be compared to see how accurately the sentinel surveillance results are describing overall adult HIV prevalence. The comparative results show that HIV prevalence among antenatal care patients is typically a suitable indicator of HIV prevalence among all 15 to 49 year olds. This means sentinel surveillance data can be used as a reasonably accurate indicator of adult HIV prevalence.10

With international assistance, many African countries are now moving into second-generation surveillance systems. These are more sophisticated efforts designed to provide quality information to inform HIV/AIDS prevention and care programs. The goals of second-generation surveillance include efforts to (a) achieve a better understanding of changes in the epidemic over time; (b) achieve a better understanding of the behaviors that are the driving factors behind the epidemic; (c) develop more focused surveillance of high-risk populations; and (d) make better use of surveillance information to plan prevention and care interventions.11

Age-Sex Distributions

Both HIV and AIDS strike hardest among people 15 to 49 years old. This is the most economically productive portion of the population, and high sickness and death rates from HIV/AIDS result in an important economic and development loss to Africa. Since these are the reproductive ages as well, it means that families are disrupted by HIV/AIDS-related sickness and death.

Sub-Saharan Africa appears to be the one region of the world where more women than men are infected. According to UNAIDS, of the 26 million infected adults 15 to 49 years old in sub-Saharan Africa, women now constitute 15 million, or 58 percent.12 Women also become infected at younger ages. More than two-thirds of newly infected 15 to 19-year-olds in sub-Saharan Africa are female. Of the 8.6 million young people ages 15 to 24 living with HIV/AIDS in the region, some 67 percent are young women.13

Conversely, few infections exist in the 5 to 14 year old age group. Delays in the onset of sexual activity among young people and partner reduction among the young can have an enormous impact on the overall epidemic. This is why programmatic efforts often place an important emphasis on working with young people.


  1. The use of antiretroviral drugs in more developed countries is now an established therapy for delaying the deterioration of the immune system among at least some HIV-infected individuals. What role antiretrovirals might play in African countries in coming years is still unclear. The cost for these drugs has dropped dramatically (though still prohibitively expensive by African standards), and public health officials would like to see them used more widely on the continent.
  2. U.S. Centers for Disease Control (CDC), “The Impact of Tuberculosis Worldwide,” accessed online at www.cdc.gov/nchstp/tb/
    worldtb2001/tbglobal.pdf, on July 19, 2002.
  3. CDC, Fact Sheet: Human Immunodeficiency Virus Type 2, accessed online at www.cdc.gov/hiv/pubs/facts/hiv2.htm, on July 17, 2002.
  4. For a good discussion of HIV virology, see UNAIDS, HIV Variability. This document can be found at www.unaids.org/hivaidsinfo/faq/
  5. Joint United Nations Programme on HIV/AIDS (UNAIDS), Report on the Global HIV/AIDS Epidemic: July 2002 (Geneva: UNAIDS, 2002). Of course, some infections are transmitted by other means in Africa, but the total is negligible compared to the number resulting from heterosexual contact.
  6. The incubation period for children is much shorter.
  7. See, for example, John Stover, AIM, Version 4: A Computer Program for Making HIV/AIDS Projections and Examining the Social and Economic Impact of AIDS (Washington: The POLICY Project, 1999): 20.
  8. UNAIDS, AIDS Epidemic Update – December 2001 (Geneva: UNAIDS, 2002): 17.
  9. Other groups may be sampled as well. In many sentinel systems, blood samples are also taken from sexually transmitted infection patients. STI patients serve as a good indicator of HIV prevalence levels in a high-risk population, but they are not a good indicator of prevalence in the overall population.
  10. Sentinel surveillance tends to overestimate prevalence among 15 to 49 year old women and underestimate prevalence among 15- to 49-year-old men. It also does not necessarily accurately reflect what is happening in each age group. But as an overall indicator of adult HIV prevalence, the sentinel surveillance results are remarkably close to the population-based survey findings.
  11. Leading the Way: USAID Responds to HIV/AIDS (Washington: TvT Associates, Inc., 2001): 33.
  12. UNAIDS, Report on the Global HIV/AIDS Epidemic: July 2002 (Geneva: UNAIDS, 2002).
  13. United Nations Children’s Fund (UNICEF), Young People and HIV/AIDS: Opportunity in Crisis (New York: UNICEF, July 2002)

Thomas Goliber is a senior fellow at Futures Group International, an international development organization headquartered in Washington, D.C. and Bath, United Kingdom. He is a reproductive health policy specialist with expertise in sub-Saharan Africa, a region where he has been working for more than two decades.