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AIDS is Acquired Immume Deficiency Virus and it is caused vy the human immunodeficiency virus (HIV). Viruses are not like living cells such as bacteria and fungi which can absorb nutients, break them down into their components, and use the energy trapped in these components to run all their cellular reactions including those used for replication. So when disease-causing or pathogenic bacteria invade our bodies, they can absorb nutrients from their local environment, use these to multiply, and therby cause disease either locally, for example in the middle ear causing earache (otitis media) or systemically by invading the general blood circulation which happens in bacterial meningitis.

Viruses are not cells, so are not capable of "living" in the sense of being independent self-replicating organisms. They get around this problem by invading cells, taking over the host (invaded) cells' DNA replicating factory, then using it to produce not only their own DNA, but also all their other constituent parts, which are then assembled in the cell. As a finishing touch, when they have assembled enough virus particles, they burst the cell (destroying it in the process) and the virus particles are released to repeat the cycle by invading more cells.

Viruses which cause disease in humans are packets of protein capsules which contain usually DNA. So when they invade human cells, the viral DNA, if it is in the correct form, can fuse with human DNA, then when triggered into action, can start to make proteins and enzymes which take over the cell. HIV is unusual because it has RNA (ribonucleic acid) instead of DNA as it's genetic material, and is called a retrovirus. So for HIV, it has do do all that DNA containing viruses do for replication, but HIV also has to convert RNA into DNA as well. In order to do that, it has a special and unique enzyme which it makes called reverse transcriptase (or RT). Now for scientists, when they find things which are special and unique in pathogens, they get excited because it means they have found a weakness that can be exploited.

HIV is so deadly because the cells which it invades, the helper T cells of the immune system, have such an important role in controlling and co-ordinating the body's defense mechanisms. It would be the equivalent to a deadly and decimating attack on the Pentagon during a war. The tanks and troops are numerous and ready for combat, but the command centre has been wiped out. HIV achieves this by a special attachment on the

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