Good health requires a good physical and social environment, good economic conditions, and social equality and harmony. When a person falls ill, their normal bodily functions or appearance change for the worse, resulting in symptoms. Doctors examine these symptoms to identify the disease and conduct tests to confirm the diagnosis. Diseases can be acute and short-term, chronic and long-lasting, infectious and spread between people, or non-infectious. They are caused by pathogens, poor nutrition, bodily malfunctions, environmental pollution, or genetic disorders.