How Does HIV Cause AIDS?

The HIV virus predominately infects two types of immune cells, helper T-lymphocytes and monocytes, targeting the CD4 protein carried on the surface of those cells. The result is that over a period of years those CD4+ (CD4 positive) cells decline to such low levels the immune system can no longer function effectively. Not only is the patient susceptible to new infections a whole host of normally harmless passenger micro-organisms (viruses, bacteria, and fungi) can no longer be restrained and as a result cause overt disease.