An illness is what you have when you are sick. This is always a bad thing and ranges on a scale of unpleasantness from the bother of a runny nose to the horror of bits falling off. Most illnesses are caused by an infectious agent (not to be confused with PPC agents, bar one exception) or an inherited condition. Many illnesses can and do result in death, some quickly, some slowly.
Many agents view Mary-Sues as an infection and themselves as the cure.