This is an electron microscopic photograph of a CD4 lymphocyte, the main target cell of HIV infection, to which many blue-colored virions are attached. Attachment occurs by interaction of HIV's gp120 glycoprotein with the cell's CD4 receptors.

• HIV primarily infects CD4+ T- lymphocytes.
• Continuous viral replication occurs throughout the course of HIV disease.
• Up to 10 billion virions are produced and cleared daily.
• The half-life of an HIV-infected CD4 cell is about 1.3 days.
• Most CD4 cells turn over rapidly, but some belong to a latent pool with a long half-life.
• Virus-specific CD4 cells, which are critical in maintaining an effective host immune response in chronic viral infections, are present early in HIV infection but are generally lost over time.
• The patient's immune system keeps pace with this activity during the clinical latency period.
• However, in the absence of effective antiretroviral treatment, the immune system ultimately reaches a "point of exhaustion" at which viral replication exceeds its ability to produce CD4 cells; this leads to a decline in immunologic function and the development of clinical disease manifestations, including opportunistic infections and neoplasms.
• The rate of viral replication is thought to stabilize after primary infection at a particular level or "set point"; this level may be maintained within a ten-fold range over months and possibly years; the viral load is highly correlated with the rate of disease progression and mortality.