AIDS is caused by the > Human Immunodeficiency Virus (HIV)
The virus infects and destroys the cells of the body's immune system causing a decrease in their number.
The cells destroyed are called > T helper lymphocytes. Without enough of these, the body is unable to defend itself and a range of pathogens are able to cause a variety of so-called > opportunistic infections.
The types of opportunistic infection that AIDS sufferers aquire are illustrated in the figure below:
Because HIV is an infective agent AIDS is called an 'aquired' immunodeficiency to distinguish it from other types such as the inherited form.
Geographic Distribution
After initial uncertainty in the 1980's at the emergence of an apparently new disease it soon became clear that an epidemic and then a pandemic were underway.The disease is worldwide, with some 40 Million estimated sufferers in 2001. The incidence (new sufferers) is estimated to be more than 5 ,illion per year.
It has been estimated that 3 million people died in 2001 from AIDS, and as the diagram below demonstrates, the majority of these died in sub-Saharan Africa. More than 34 million people are estimated to have died in the region from AIDS. It has also been estimated that 80% of the world's deaths from AIDS occur in Africa. The population of Zimbabwe is reckoned to be 25% infected. AIDS is having a major impact on the economies of these countries. It affects sexually active people in their 20's and 30's, and it is these individuals who are potentially the most economically productive.
The Virus and Infection
HIV is a virus and it cannot survive outside the human body. There is no vector, and it is spread by intimate human contact. Below is a electron micrograph of the virus, together with a diagrammatic representation of the virus.
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The virus is spread through the direct exchange of body fluids. It is spread most easily through 4 mechanisms:
- > Sexual intercourse
- > Blood donation
- > Sharing needles
- > Across the placenta to the fetus
You do not need to know the way in which the HIV virus 'hijacks' the white blood cells in order to make more copies of itself, but if interested click here for a simplified diagrammatic representation.
HIV is slow, and after infection there may be no symptoms whatsoever for years, even a decade. Such people are HIV positive, but do not have AIDS. The opportunistic infections that characterise AIDS tend to include oral thrush and pneumonia. As the system gets progressively less useful, rare cancers called > Kaposi's sarcomas, of the skin and internal organs.
Treatment and Control
There is no cure for AIDS and no vaccine for HIV. There is of course much interest in vaccines, click here for an exploration of the idea. As the virus changes its surface proteins it is very hard for the body's immune system to recognize it.
Drug treatment can slow down the onset of AIDS massively, and many in developed countries it has extended the life expectancies of many sufferers greatly. The drugs are similar to DNA nucleotides.
How does Zidovudine work? > It binds to the viral enzyme reverse transcriptase and blocks its action. This stops the viral genetic material from replicating and leads to an increase in some of the body's white blood cells.
The drugs often have to be taken in a complicated regime, and failing to follow it can lead to the development of drug resistant strains. The drugs do have side effects however, ranging from headaches and rashes to permanent nerve damage. The cost of effective treatment is great however, and such treatment is outside the economic abilities of developing nations to afford.
Avoiding HIV is about avoiding the exchange of body fluids, as detailed in the somewhat severe tone of the table below:
Contact Tracing This is an important part of HIV control. Basically, if someone who has been recently diagnosed is willing, all those with whom they have had sexual intercourse or shared needles are offerered a test.
Needle Exchange Drug users are urged to give it up (!) , stop sharing needles, or to take their drugs in other ways. In some areas needle exchange schemes exist.
Blood Screening In developed countries, blood is routinely screened for HIV.In developing nations such procedures are not followed, and people are advised to donate some of their own blood in advance of an operation.
Testing In developed countries, HIV testing is promoted to high risk groups like male homosexuals, drug users and prostitutes. If they test positive they are given medical and psychological support.
In developing nations the disease is not so simply identified to such easily identifiable groups. HIV+ people in such nations tend to discover that they are infected only when they develop AIDS
Can you complete the summary table below?
| Feature | Description |
| Pathogen | > Human Immunodeficiency Virus |
| Method of transmission | > In semen and vaginal fluid during sex, infected blood, contaminated syringes, across the placenta, to an infant in breast milk |
| Global distribution | > Worldwide, especially in sub-Saharan Africa and SE Asia |
| Incubation period | > initial incubation a few weeks. It may take up to 10 years or more for AIDS to develop |
| Site of action | > T helper lymphocytes, macrophages, brain cells |
| Clinical features | > HIV infection- flu-like then nothing AIDS- opportunistic infections including TB, cancers, pneumonia, diarrhoea |
| Method of diagnosis | > blood test for HIV antibodies |
| Number of HIV sufferers | > 40 million in 2001 |
| Worlwide annual incidence | > 5+ million |
| Worldwide annual mortality | > 3 million |