Health is defined as complete physical, mental and social well-being, while illness is any situation that impairs one's health. Infectious illnesses are caused by microorganisms like fungi, bacteria, or viruses entering the body through contaminated water, air, or open wounds, whereas non-infectious illnesses can result from factors like poisoning, accidents, poor eating habits leading to conditions like obesity and heart disease, abnormal cell growth causing cancer, and fatigue/anxiety causing sleep disorders.