This document provides an overview of health and diseases. It defines health as a state of complete physical, mental and social well-being. Ancient beliefs held that health was influenced by four bodily fluids or "humors", but this was later disproven. Modern conceptions recognize the influence of genetics, lifestyle, infections and other factors. Diseases can be congenital or acquired, infectious or non-infectious. The immune system protects against disease through innate and acquired immunity. Vaccines help prevent infectious diseases by stimulating immune memory. Allergies represent inappropriate immune responses.