Your immune system contains different types of cells that help protect the body from infection. One of these types of specialized cells are called the CD4 or T-cells. HIV attacks these types of cells and uses them to make more copies of HIV. And in doing so, HIV weakens the immune system, making it unable to protect the body from illness and infection.
Early in the course of the disease, the body can make more CD4 cells to replace the ones that have been damaged by HIV. Eventually, the body can't keep up and the number of functioning T-cells decreases. As more and more CD4 cells become damaged, the immune system becomes more and more weakened. Eventually, the weakened immune system leaves the body at risk for illness and infection infections.
On a regular basis, your doctor will draw blood to measure the number of functioning CD4 cells. The higher the number the stronger your immune system. People without HIV infection have about 700 to 1000 CD4 cells in a drop of blood the size of a pea. HIV infected people are considered to have "normal" CD4 counts if the number is above 500 CD4 cells in that same size drop of blood.
If the number of CD4 cells in that drop of blood ever drops below 200 CD4 cells, you are classified as having AIDS. Simply put, experts have found that when your CD4 count drops below 200, your body's immune system is no longer strong enough to prevent illness and infection. Some of the most serious of these illnesses and infections are said to be AIDS defining.