HIV
A retroviral infection that progressively destroys CD4 T-helper cells, so that disease is defined not by the virus itself but by the opportunistic infections and cancers that emerge once immune surveillance collapses.
First principles
HIV is a disease of progressive CD4 T-cell depletion
HIV preferentially infects and destroys CD4-positive T-helper cells, using CD4 together with a co-receptor to enter the cell. These cells coordinate both cell-mediated and humoral immunity, so ongoing viral replication kills them faster than they can be replaced, and the CD4 count falls steadily over years unless treatment intervenes. The CD4 count is therefore a direct measure of how much immune coordination capacity remains, and it is this progressive depletion (not the virus's direct tissue damage) that produces disease.
Educational content pending clinical review. Not medical advice.