Why don't natural killer cells destroy normal body cells?

Why do NK cells not kill RBC?
The “self” MHC receptors of cells would normally act as an inhibitor. When a cell (such as red blood cell) gets infected, the pathogen's receptor (antigen) will tend to act as an activating receptor on NK cells. ... So unless RBC sends a signal telling NK cells to act on them, it won't kill them.

When a virus infects a host cell, it uses the cell's machinery to make copies of its own genetic material in a process called replication. finally, during the process of self-assembly, that protein shell and the copies of the virus's genetic material are assembled into new viruses.