After an individual is infected, a process begins by which the virus slowly destroys the host's immune response. The progression of events is now well defined and its most significant co-factor is simply time. Given time, and the absence of effective treatment, the virus will eventually render an infected person incapable of mounting an immune response to a number of different infections, eventually resulting in death.

This graph is critical to understanding the effect of HIV on the immune system from point of infection onward, as shown in an individual monitored over an 83-month period. The virus slowly destroys CD4+ lymphocytes until virtually none remain, eventually making it impossible for the body to defend itself against even the most common infections.

Once the virus infects the CD4+ lymphocytes it becomes part of the cell's genome and turns it into a virus-producing factory. In the infected host, billions of viral particles are produced daily. Although the infected lymphocytes are killed in the process, the released virus enters uninfected cells and the process begins again. The virus can remain latent in the infected person's memory CD4+ lymphocytes allowing it to survive in the host for many years. HIV affects other white blood cells differently. Although it infects and destroys the CD4+ lymphocytes, infected macrophages and monocytes survive as virus reservoirs. The continued destruction of the CD4+ lymphocytes, the first line of the body's immune defense system, ultimately renders the individual susceptible to otherwise non-life-threatening infections such as pneumocystis carinii pneumonia or toxoplasmosis.

This illustrates the course of the virus infection (solid line) and the body's immune response (dotted line). Shortly after infection the body mounts a strong immune response to the virus, but over time the virus is able to eventually destroy an individual's ability to produce antibodies. This argues for the concept of dynamic infectivity, wherein an individual becomes more infectious the longer he or she is infected with increasing amounts of virus found in body fluids.

Early in the epidemic clinicians speculated that the longer an individual was infected, the more infectious they became. This theory, called "dynamic infectivity", has now largely been proven. Over time, absent antiretroviral treatment, increasing quantities of free virus are found in the blood along with decreasing numbers of CD4+ lymphocytes. It is now recognized that infectivity increases directly with increasing levels of virus in the blood. Consequently, learning not only one's HIV status, but also one's particular stage of infection can be helpful in deciding on treatment as well as in controlling viral transmission.