Afflicted means "impaired" or "stricken" and usually refers to a person who is mentally or physically unfit, or has been grievously affected by disease.
This adjective's Latin root, afflictare, means "to damage, harass, or torment," which may sometimes be a good description of how an afflicted person feels. Those with mental illness may be tormented by voices in their head and thus be unable to distinguish what is real from what is not. Those afflicted by ALS, or Lou Gehrig's Disease, slowly lose control of their muscles and become paralyzed.