The basic defect in AIDS is that the virus destroys the immune system of the affected individual. In fact, it is not as much the virus as the individual who is responsible for all the illnesses suffered as a result of AIDS. All that the virus does
is rendering the immune system impotent, thereby affecting the individual's capacity to ward off common infections. This is why pneumonia happens to be the leading cause of mortality in AIDS. One thing to be noted is that being HIV-positive does not necessarily mean having AIDS. These two are different scenarios. Full-blown AIDS occurs only after certain immune functions are completely destroyed by the virus.