In 2000, 20 years after the appearance of AIDS, it had become the deadliest disease in the world. HIV causes harm to the body mainly through its effect on the immune system. HIV greatly reduces or sometimes destroys the body's immune system, which is responsible for fighting disease. Mild changes in the immune system can usually be seen very shortly after infection with HIV. They usually do not cause any symptoms. CD4, or "helper T-cells," are white blood cells that are the primary target of the HIV virus. These cells decrease steadily with the advance of disease. A patient with low levels of these cells are at high risk of contracting infections that normally would be controlled easily by the immune system.
It is not known why HIV causes faster damage to the immune system in some people compared to others. There are different stages of HIV disease which match the changes in the immune system. In the beginning the person is infected with the virus and may have no signs of sickness. They are said to be asymptomatic. Next, symptoms develop that are first mild and then become more and more severe (the person is said to be symptomatic). At some point, a person will be diagnosed as having "AIDS". This is usually the most severe form of HIV infection. A person is given a diagnosis of Acquired Immune Deficiency Syndrome (AIDS) when they have developed specific problems due to HIV.