AIDS, or Acquired Immunodeficiency Syndrome, occurs when an individual's immune system is weakened by HIV to such an extent that the individual develops one or more of about 25 "opportunistic infections" (OIs), conditions that take advantage of a weakened immune system. When this happens, a person who is HIV positive is considered to have developed AIDS, or to have an "AIDS diagnosis". They are also considered to have an AIDS diagnosis when their CD4 cell count (a special type of white blood cells that fight infection) falls below a certain level and/or the amount of virus in their body rises above a certain level.