White blood cells, which are produced in the bones, are a major component of the body's immune system. When an infectious organism such as a virus or bacteria enters the body, the white blood cells are the first line of defense for destroying the invading organism. Infection with certain organisms, however, can have serious effects on the immune system. AIDS, or acquired immune deficiency syndrome, is a disorder that is caused by infection with the human immunodeficiency virus (HIV). This virus attaches to the surface of specific white blood cells called T cells and is therefore able to reproduce and continue production of the virus. As more and more of the body's healthy T cells become infected with HIV, the body's immune system becomes compromised. As a result, if another foreign organism enters the body, the white blood cells are unable to fight the new infection and serious illness can result. In addition, there are many illnesses that persons infected with HIV are more susceptible for developing than are persons with healthy immune systems. A diagnosis of AIDS is made after T cell counts reach a dangerously low level and blood tests confirm the presence of antibodies to HIV, indicating that the virus is present in the body. Symptoms of AIDS may not appear for 5 to 10 years after being infected with HIV.