This section covers several topics which relate directly to AIDS. In order for one to understand AIDS, one must understand the topics explained below. The following information is by no means exhaustive, but it is enough to provide one with the basic idea of each topic.
The Immune System
HIV causes AIDS by destroying a person's immune system. The HIV virus attacks one type of immune cells in particular: the T-helper cells. These cells assist in antibody production. Antibodies are molecules that attach to a specific antigen (any foreign material in the body) and target it for destruction. The T-helper cells also trigger several other parts of the immune system. As the disease progresses, the number of these cells decreases dramatically, which almost completely shuts the immune system down. HIV also attacks the organs of the immune system, further weakening it.
Genetic material is what determines the structure and function of all living things. There are actually two types of genetic material: RNA and the much more famous DNA. The main structural difference between the two is that RNA is a single-stranded molecule, while DNA is a double stranded molecule. Because of this difference, RNA is much more prone to mutations (spontaneous changes) than DNA. In most organisms, RNA plays a secondary role to DNA. However, in some viruses, including the HIV virus (the one that causes AIDS), RNA is the only genetic material present.
In order to understand how the AIDS virus infects the body, one needs to know a little bit about viruses and viral reproduction in general. Viruses are the simplest organisms on the planet. They are nothing more than genetic material surrounded by a coat of protein. They aren't composed of cells, and, therefore, can not reproduce on their own. In order to reproduce, viruses need to use a cell of another organism. The virus uses the other cell by "injecting" it with the virus' DNA. This DNA effectively retools the cell and changes it into a viral factory. As the cell produces more and more viruses, it begins to swell, since the viruses can't leave the cell once they are created, and eventually, bursts. When the cell bursts, all of the viruses it created are set free to infect more cells.