Everything smells. What you mean is the particular, overpowering smell of death. This is the process of decomposition where various gases are emitted from microorganisms. The trapped gases from bacteria in the intestinal tract in the second stage of decomposition cause the worst smell. The smell can permeate some materials and this cannot be removed by ordinary household cleaning products.
The smell is abhorrent for all but vultures who are drawn to eat dead meat. This automatic feeling of revulsion from the smell is probably evolutionarily important, keeping us from contamination where someone has died of disease.
