HIV and AIDS

HIV, the Human Immunodeficiency Virus, is a retrovirus — meaning its genetic material is RNA rather than DNA, and it carries a special enzyme, reverse transcriptase, that lets it convert its RNA into DNA once inside a host cell. Structurally, HIV consists of an outer envelope studded with glycoprotein spikes that allow it to recognise and bind to specific target cells, surrounding an inner protein core that encloses two copies of its single-stranded RNA genome along with its own reverse transcriptase enzyme.

HIV specifically targets and infects helper T-lymphocytes (also called CD4+ T-cells) — precisely the cells described in section 7.3 as the coordinators of the body's acquired immune response. Its replication cycle proceeds through a defined sequence of steps: the virus first attaches to and enters a helper T-cell; inside the cell, its reverse transcriptase enzyme converts the viral RNA into viral DNA; this viral DNA is then incorporated (integrated) into the host cell's own DNA; using the host cell's machinery, new viral RNA and proteins are produced; and finally, new virus particles assemble and bud out from the host cell's surface, ready to infect further helper T-cells — a process that, in doing so, progressively destroys the very cells the immune system depends on.

As the number of helper T-cells declines over time, the body's ability to mount an effective immune response against other infections weakens correspondingly. This progressive collapse of immune function is what eventually produces AIDS (Acquired Immunodeficiency Syndrome) — the advanced stage of HIV infection in which the severely weakened immune system leaves a person vulnerable to infections and cancers that a healthy immune system would normally control without difficulty.

HIV spreads through a limited number of well-defined routes: unprotected sexual contact with an infected person, transfusion of contaminated blood or blood products, sharing needles or syringes (particularly among people who inject drugs), and from an infected mother to her child during pregnancy, childbirth, or breastfeeding.