But recent research shows that the virus is never resting, or dormant. In fact, the immune system struggles continuously against the virus, which grows quickly from the first moment of infection. The human body is under continuous attack by a wide variety of bacteria, viruses, and fungi. The immune system's job is to remove or control these intruders and to protect the body from their damaging effects. One of the immune system's most important parts is called a CD4 cell or a T cell.
T cells lead the immune response against infections.
Specifically, they direct other cells to find and destroy bacteria, fungi, and viruses that cause infections. Once HIV invades T cells, T cells can't be protective leaders of the immune system. The constant attack and growth of the virus, targeted mainly at the T cells, gradually wears down the immune system and weakens the body's defenses against bacteria, viruses, and fungi. When this happens, opportunistic infections can develop. They are called opportunistic infections because they occur only when damage to the immune system presents an opportunity.