When the immune system collapses, ordinary infections become lethal. AIDS, or acquired immunodeficiency syndrome, is the advanced stage of infection with the human immunodeficiency virus in which the immune system is severely damaged and opportunistic diseases appear.

HIV progressively destroys CD4 T-cells. Once the count falls low enough or certain indicator illnesses develop, the diagnosis of AIDS is made. Modern antiretroviral therapy can suppress the virus, restore immune function, and prevent progression for decades. Without treatment the natural course is progressive decline and death from infection or cancer.

Transmission occurs through blood, sexual contact, and from mother to child. Prevention, early testing, and continuous treatment have transformed AIDS from a rapidly fatal diagnosis into a manageable chronic condition for most people with access to care. Global programs continue to expand that access.

• Advanced stage of HIV infection with severe immune damage
• Marked by opportunistic infections or very low CD4 counts
• Preventable and controllable with antiretroviral therapy
• Transmitted by blood, sex, and perinatal routes