Human immunodeficiency virus (HIV) affects the immune system by the destruction of human T-helper cells, or CD4 cells. A significant loss of CD4 cells depletes the body's ability to protect itself from “opportunistic infections,” creating the condition of acquired immune deficiency syndrome (AIDS), usually when the CD4 count falls below 200 cells/mm. The time from HIV infection to death without the benefit of antiretroviral therapy is currently 10–11 years in the average patient. Age as a host factor influences the rate of HIV disease progression. Aging is associated with a higher viral load following seroconversion, more aggressive disease progression, shorter survival rates, and increased intolerance of antiretroviral agents. Physiological changes of aging such as involution of the thymus gland may inhibit function of CD4 cells, and CD4 cell regeneration generally slows with age. There are other common changes with aging that can affect disease progression. There is an increased risk of autoimmune disorders, a decrease in renal and lung function, neurological and psychological changes (such as depression and dementia), and postmenopausal vaginal changes. Additionally, comorbidity and poor nutrition could affect disease progression.

Although the term “elderly” is usually applied to patients in the sixth decade, the Centers for Disease Control and Prevention (CDC) defines elderly AIDS patients as those 50 years and older. AIDS cases in persons older than 50 years, originating before 1989, were a result of contaminated blood transfusions from 1978–1985, representing 6%, 28%, and 64% of AIDS among persons aged 50–59 years; 60–69 years; and older than 70 years, respectively.