A disease is an abnormal condition that negatively affects the structure or function of all or part of an organism, impairing its normal functioning. It is not immediately caused by external injury but often involves a disruption of the body's homeostasis, or internal balance
. Diseases can manifest with specific signs and symptoms indicating the abnormal state of the organism
. Diseases may be caused by external factors such as pathogens (viruses, bacteria, fungi, parasites) or by internal dysfunctions like immune system disorders, genetic defects, or nutritional deficiencies
. They can affect physical or mental health and vary in severity from minor and temporary to serious and chronic conditions
. In humans, disease broadly refers to any condition that causes pain, dysfunction, distress, or social problems, and includes infectious diseases, deficiency diseases, hereditary diseases, and physiological diseases
. The study of diseases, including their causes (etiology), development (pathogenesis), and effects, is called pathology
. In summary, disease is a condition that disrupts normal bodily functions, often indicated by symptoms, and can arise from a variety of causes both external and internal
