The mechanism by which HIV does cause illness, is damaging the immune system. It can infect many human cells, but white is the CD4 lymphocyte (also known as CD4 cell, cell ayudante-T or cell helper). The CD4 cell is a type of blood cell white, responsible for control or prevent diseases caused by a variety of common viruses, bacteria, fungi and parasites and even some types of cancer. HIV infection leads to the destruction of CD4 cells and eventually the count decreases, to such an extent that the patient cannot fight infection and is there where symptoms and complications appear; However, reaching that point can take years. The pace in which the CD4 cells decline varies from person to person and depends on multiple factors, including the characteristics of the viral strain, genetic characteristics and the amount of virus in the blood (viral load).

The cause of the loss of CD4 cells still not completely understood. It is not a simple question that HIV infects and kill the cells, because the amount of infected cells is low. Scientists now believe that HIV can cause a chronic immune activation (stimulation of the immune system) that leads to a reduction in the number of CD4 cells. Besides the damage to the immune system, HIV can directly affect many organs of the human body, such as the nervous system and the kidneys. It can also cause loss of weight, night sweats and diarrhoea. When the AIDS deaths were common, it was said that the person was not dying HIV as such, but because of the complications associated with it, such as cancer or infections. While in some cases this may have been technically true, mostly, HIV infection is still the fundamental problem that leads to death by AIDS.