Why can a person with AIDS die from a simple flu that would not kill a healthy person?
The common infections, such as cold, flu, etc. are fought by the immune system of our body. Specifically, the white blood cells or WBCs are the fighters against foreign infections.
When a person becomes infected with HIV or human immunodeficiency virus, the count of these fighters start decreasing. More specifically, HIV starts destroying the CD4+ T cells, thus damaging the body's natural immunity. A healthy human being contains 600-1200 CD4+ T cells per cubic millimeter of blood. However, this count starts falling when the body is infected with HIV.
AIDS or acquired immunodeficiency syndrome is the final stage of HIV infection. An AIDS patient has less than 200 CD4+ T cells per cubic millimeter of blood. At this stage, the body's natural immunity is more or less destroyed and simple infections are fatal. A healthy human body (HIV negative) can fight these infections easily, an AIDS patient cannot.
Kindly note that HIV infection is different from AIDS (it is only the advanced stage of infection). An HIV-positive person can live for a number of years before being diagnosed with AIDS.
Hope this helps.