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Epidemiology of Infectious Diseases: HIV/Aids

Causal agent
Human immunodeficiency virus (HIV), a retrovirus is the causative agent for Acquired Immunodeficiency syndrome (AIDS).  

Two serologically and geographically distinct types, HIV-1 and HIV-2 have been identified.

Both HIV-1 and HIV-2 have the same modes of transmission and are associated with similar opportunistic infections and AIDS.

HIV-1, which is responsible for the majority of AIDS cases worldwide, is divided into three groups - the 'major' group M, the rarer 'outlier' group O and 'new' group N.  Within the M group (which accounts for up to 90% of HIV infections worldwide), at least 9 strains (clades) of HIV-1 have been identified. 

HIV-2 infections are less common and are predominantly found in West Africa. HIV-2 infection has a longer latent period before the appearance of AIDS, a less aggressive course of AIDS and a lower viral load with higher CD4 lymphocyte counts thanHIV-1 infection until late in the course of the disease when clinical AIDS is apparent.

Common clinical features
The clinical manifestations of HIV infection range from the initial acute retroviral syndrome to full blown AIDS1.

Acute retroviral syndrome (ARS) is the first stage of infection with the human immunodeficiency virus (commonly occurring between 1-6 weeks following infection) and is characterized by an acute self-limited mononucleosis-like illness lasting for a week or two2.  However, primary infection may go unnoticed in up to 50% of cases.

Following exposure there is a period of viraemia during which the individual is highly infectious. Generally, within 3 weeks to 3 months following infection the immune response is accompanied by a simultaneous decline in HIV viraemia.

The stage of clinical AIDS is reached years following infection and is marked by the appearance of one or more of the typical opportunistic infections or neoplasms diagnostic of AIDS by definitional criteria1.

Untreated, half of those with HIV infection will develop AIDS within 7-10 years and of these 80-90% will die within 3-5 years2.  

Data from the UNAIDS 2006 Report of the Global Aids Epidemic report that in 20053.

  • An estimated 38.6 million people worldwide were living with HIV.
  • An estimated 4.1 million people became newly infected with HIV.
  • An estimated 2.8 million people lost their lives to AIDS.
  • The HIV incidence rate is believed to have peaked worldwide in the late 1990s, and to have stabilised (with the exception of increasing incidence in a number of countries).

Table 1. Estimated number of HIV infections and new infections of HIV in 20


Adults (15+) and children infected with HIV in 2005

Adults (15+) and children newly infected with HIV in 2005


(15-49) HIV prevalence (%)

Adults (15+) and child deaths due to AIDS in 2005






Sub-Saharan Africa

24.5 million

2.7 million


2.0 million






North Africa/ Middle East

440 000

64 000


37 000







8.3 million

930 000


600 000







78 000









Latin America

1.6 million

140 000


59 000







330 000

37 000


27 000






Eastern Europe & Central Asia

1.5 million


220 000


53 000






North America, Western & Central Europe

2.0  million

65 000


30 000


38.6 million

4.1 million


2.8 million


[33.4 - 46.0]

[3.4 - 6.2]

[0.9 - 1.2]

[2.4 - 3.3]

Source: UNAIDS - 2006 Report on the Global AIDS Epidemic3.

Sub-Saharan Africa

  • Sub-Saharan Africa (SSA) remains the worst affected region in the world. An estimated 24 - 27 million people in sub-Saharan Africa are infected with HIV representing 64% of all global HIV infections.
  • The prevalence of HIV infection in Africa varies significantly between and within sub-regions and countries.
  • 9 out of 10 children (<15 years) infected with HIV worldwide live in SSA.
  • 75% of all women (>15 years) infected with HIV live in SSA, where in most of the region women are disproportionately affected by AIDS (on average, 3 women are HIV-infected for every 2 men. In the 15-24 year age group the ratio increases to 3 women to every 1 man infected.
  • Southern Africa remains the global epicentre of the epidemic. An estimated 1 in 3 people infected with HIV globally live in this region.
  • While recent declines in HIV prevalence have been noted in Kenya, Zimbabwe and urban areas of Burkina Faso, elsewhere in Southern Africa there are no clear signs of declining HIV prevalence.
  • Notably high infection rates are reported in South Africa, Botswana, Namibia and Swaziland. In 2005 national adult HIV prevalence in Botswana was estimated to be (24.1%), Lesotho (23.2%), Swaziland (33.4%) and South Africa (18.8%).
  • West Africa is less severely affected than other parts of SSA, with national HIV prevalence estimates lower than 2% for some countries.
  • Antiretroviral therapy in SSA has increased more than 8 fold since the end of 2003. However access to therapy varies significantly across the region from over 50% in Botswana, Namibia and Uganda to <20% in South Africa.


  • An estimated 8.3 million people were living with HIV in Asia at the end of 2005. Over 70% of them in India where an estimated 5.2 million people aged 15-49 are infected with HIV.
  • In India most HIV infections are estimated to occur through unprotected heterosexual intercourse, while in some parts of Northern India injecting drug use appears to be the main mode of transmission.
  • Between 2003 and 2005, the number of people receiving antiretroviral therapy in Asia increased from 70,000 to 180,000. However, only 16% of people in need of antiretroviral treatment in Asia are now receiving it.

Eastern Europe and Central Asia

  • Between 2003 and 2005 the number people living with HIV increased by more than one-third, with the majority of cases occurring in the Ukraine and the Russian Federation (which has the largest AIDS epidemic in Europe).
  • A large proportion of HIV infections in Russia are among injecting drug users which is increasingly spreading to their sexual partners and beyond.
  • The number of women infected with HIV in Russia increased by one-third between 2003 and 2005. This trend is particularly marked among women aged 15-20 years.
  • In 2005 an estimated 5% of people in need of antiretroviral treatment were receiving it in this region.

Latin America

  • An estimated 1.6 million infections are reported from countries in central and South America.
  • Approximately 73% of people in the region have access to antiretroviral treatment. However, a marked geographical variation exists with the poorest countries in Central America and in the Andean region of South America having less access to antiretroviral treatment.
  • Over one third of people living with HIV in Latin America are from Brazil, where the highest HIV infection rates are found in injecting drug users. While the most intense epidemics are underway in Belize and Honduras.

North America, Western and Central Europe

  • An estimated 2 million people are infected with HIV in this region (1.2 million in the United States)
  • An estimated 720,000 people in Europe and Central Europe are infected with HIV, where heterosexual sex has become the main mode of transmission in several countries.

United Kingdom

  • By the end of 2004 there were an estimated 58,300 people living with HIV in the UK, of whom 34% were unaware of their infection4.
  • The number of newly diagnosed HIV infections in the UK increased from 3,851 in 2000 to 7,275 in 2004.  Of these an estimated 30% were in men who have sex with men (MSM). An estimated 60% acquired their infection heterosexually (75% of which are thought to have been acquired in Africa) and 2% through injecting drug use4.
  • While the number of HIV diagnoses in heterosexuals may have exceeded the number of diagnoses among MSM in recent years, many more MSM acquire HIV within the UK itself. An estimated 75% of all new HIV infections acquired in the UK in 2004 are thought to have been acquired through MSM4.
  • By the end of 2004 a total of 1,650 children <15 years had been diagnosed with HIV in the UK.
  • The level of anti-retroviral therapy (ARV) was recorded for 41,478 patients seen for care in the UK in 2004. Of these 64% were receiving 3 or more anti-retroviral drugs, 1.4% were receiving mono or dual therapy and 34% were not on HIV therapy (of which most were at too early a stage of HIV infection for ARV or were recently diagnosed with HIV infection4.

Humans, HIV is thought to have evolved from chimpanzee viruses2.

Mode of transmission

  • Person to person transmission through exposure to infected blood and tissues.
  • Unprotected sexual intercourse (anal or vaginal) with an infected partner.
  • The presence of a concurrent sexually transmitted infection (STI), especially an ulcerative one increases the risk of HIV transmission.
  • Transmission is especially efficient between male homosexuals in whom receptive anal intercourse and multiple sexual partners are particular risk factors1.
  • Sharing of contaminated needles or syringes (intravenous drug users).
  • Transfusion of infected blood or blood products.
  • Mother to child transmission (MTCT) - An estimated 15-35% of infants born to HIV positive mothers are infected through placental processes at birth2.
  • MTCT through breast milk.
  • Transmission via needle stick injury.
  • The transmission of HIV-2 is similar to that for HIV-1, though perinatal transmission is much less frequent.
  • Providing infected pregnant women with antiretrovirals significantly reduces the risk of MTCT.

Incubation period

  • Following exposure HIV nucleic acid sequences may be detected in the blood within 1-4 weeks following infection and HIV antibodies can be detected within 4-12 weeks1.
  • The time from HIV infection to diagnosis of AIDS has an observed range of less than 1 year to 15 years or longer. The increasing availability of anti-HIV treatment has reduced the development of clinical AIDS in most industrialized countries2.
  • For perinatally acquired HIV infection, the time to development of clinical AIDS may be shorter than in adults. Signs associated with HIV infection appear in over 80% of seropostive infants by the age of 5 months. Approximately 50% of children with perinatally acquired HIV infection are alive at 9 years.

Period of Communicability
From shortly after the onset of the HIV infection extending throughout life.

Infectivity during the first months is considered to be high; it increases with viral load, with worsening clinical status and with the presence of other STIs2.

Prevention and control

  • Public health education to reduce high risk behaviours associated with the transmission of HIV.
  • No vaccine is currently available.
  • Highly active antiretroviral therapy (HAART) reduces disease progression.


  1. Hawker J, Begg N, Blair I, Reintjes R, Weinberg J. Communicable Disease Control Handbook, Blackwell, 2005.
  2. Heymann D L, editor, Control of Communicable Disease Manual. 18th ed. American Public Health Association; 2004.
  3. UNAIDS; Joint United Nations Programme on HIV/AIDS,  2006 Report on the Global Aids Epidemic, UNAIDS, Geneva,  2006. Available online at:
  4. Health Protection Agency, United Kingdom (2006), Mapping the issues; HIV and other sexually transmitted infections in the United Kingdom - 2005, London, The UK Collaborative Group for HIV and STI Surveillance. Available online at:

Further Resources

Centers for Disease Control and Prevention. Epidemiology of HIV/AIDS, United States, 1981-2005. MMWR (Morb Mortal Wkly Rep) 2006;55(21):589-592. Available online:

Centers for Disease Control and Prevention. Twenty -Five Years of HIV/AIDS - United States, 1981-2006, MMWR (Morb Mortal Wkly Rep) 2006; 55(21):585-589. Available online:

National Institute of Allergy and Infectious Diseases - US National Institutes of Health

© CM Kirwan 2006