Acquired immunodeficiency syndrome (AIDS) applies to the most advanced stages of HIV infection. People diagnosed with AIDS include all HIV-infected individuals who have fewer than 200 CD4+ T cells per cubic millimeter of blood. In addition, there are 26 clinical conditions that affect people with advanced HIV disease. Most of these conditions are opportunistic infections that generally do not affect healthy people. Among people with AIDS, these infections are often severe and sometimes fatal because their immune systems are so impaired by HIV they can no longer respond to certain bacteria, viruses, fungi, parasites, and other microbes.