I recently sat in on an Immunology class entirely devoted to the discussion of AIDS (Acquired Immune Deficiency Syndrome). AIDS is when the T cells of the immune system are no longer present in the body because they have been taken-over by the virus, HIV (Human Immunodeficiency Virus).
There are two types of HIV: the first (type 1) is what has caused the AIDS pandemic as it spreads quickly and is fast acting in its host. The second (type 2), however, is less virulent (meaning it is not as good at infecting (so it is harder to catch)) and it kills the host slowly. People with type 2 HIV live longer. According to Immunology prof, this virus is smarter because it is pretty dumb to kill the host human as the human is needed for the virus' survival!
HIV is such a destructive virus because it uses its host cells' (CD4 T cells') DNA to help it replicate itself. It copies its RNA genome into a cDNA strand, which--using host DNA replication machinery--replicates and makes more viral proteins. As such, CD4 T cell numbers diminish, and since T cells are responsible for immunity, the quality of immune system also diminishes. Only when you have 200 CD4 T cells left are diagnosed as having AIDS (even though you'll start to show symptoms of AIDS when your CD4 T cell numbers are between 200-500). As an aside, evil insurance companies didn't want to pay for the medication required to treat people all of the people suffering with AIDS, so they lobbied the government to change the original definition of AIDS from having less than 500 CD4 T cells to now having less than 200 CD4 T cells. Absolutely evil :(
When screening for HIV, blood work shows whether or not you have the protein gp120, which is an HIV protein that binds to the surface of CD4 T cells. Although unsafe sex is commonly considered the main reason for the contraption of HIV, another major reason is due intravenous drug use and unsafe needle exchange practices.