The human immunodeficiency virus (HIV) targets and destroys specific cells of the immune system, particularly the CD4 T-lymphocytes, which are central to fighting off infections. When left untreated, the destruction of these immune cells progressively weakens the body’s defenses. Acquired immunodeficiency syndrome (AIDS) represents the most advanced stage of an HIV infection, characterized by severe immune system damage. This damage makes the body vulnerable to serious diseases that a healthy immune system would normally control.
Defining the Progression to AIDS
The formal diagnosis of AIDS is based on specific clinical criteria established by public health organizations, signifying a profound state of immune suppression. The primary numerical marker for this severe immune damage is the count of CD4 T-cells in the blood. A diagnosis of AIDS is assigned when the CD4 T-cell count falls below 200 cells per cubic millimeter of blood. This number is far lower than the range of 500 to 1,600 cells/mm³ seen in healthy individuals.
A diagnosis of AIDS can also be made if a person with HIV develops one of the specific conditions known as AIDS-defining illnesses, regardless of their current CD4 count. These illnesses are manifestations of the body’s inability to fight off pathogens due to the virus’s damage. Therefore, an AIDS diagnosis is determined by either the low CD4 count or the clinical evidence of a defining illness.
Categories of AIDS-Defining Illnesses
AIDS-defining illnesses are a group of opportunistic infections and certain cancers that take advantage of the severely weakened immune system. These conditions are caused by infectious agents that are generally harmless to people with intact immune function. The presence of one of these diseases signals that the HIV infection has progressed to its most severe stage.
Many of these defining illnesses are caused by fungal pathogens. Pneumocystis pneumonia (PCP) is a common fungal infection of the lungs that rarely affects healthy people but can be deadly in individuals with AIDS. Extrapulmonary cryptococcosis is a definitive sign of AIDS when spread outside the lungs. Candidiasis of the bronchi, trachea, or esophagus, a severe form of yeast infection, also falls into this category.
Viral infections are another significant group of AIDS-defining conditions. Cytomegalovirus (CMV) can cause disease in various organs, including retinitis in the eye, which can lead to vision loss. Progressive multifocal leukoencephalopathy (PML) is a severe neurological disorder caused by the JC virus, which causes progressive damage to the brain. Chronic ulcers caused by the Herpes simplex virus lasting longer than one month are also used in the diagnosis.
Specific bacterial infections and cancers also define the syndrome. Mycobacterium Avium Complex (MAC) is a type of bacterial infection that can disseminate widely throughout the body in people with AIDS. Tuberculosis (TB), particularly when it occurs outside the lungs, is an AIDS-defining condition. Certain malignancies, such as Kaposi’s sarcoma, and specific types of Non-Hodgkin Lymphoma, are strongly associated with advanced immune suppression.
Preventing Illnesses Through Antiretroviral Therapy (ART)
The modern approach to managing HIV infection focuses on preventing the immune system from ever deteriorating to the point where these defining illnesses can take hold. Antiretroviral therapy (ART) is a combination of medications that suppresses the replication of the HIV virus. This therapy dramatically lowers the amount of HIV in the blood, known as the viral load.
By keeping the viral load suppressed, ART allows the body’s immune cells, particularly the CD4 T-cells, to multiply and restore their numbers. This restoration of immune function prevents the progression to AIDS. A robust immune system can effectively control the opportunistic pathogens that cause AIDS-defining illnesses. For people living with HIV who take their medication as prescribed and achieve an undetectable viral load, the progression to AIDS has become increasingly rare.