Disease

A disease is a particular abnormal condition that adversely affects the structure or function of all or part of an organism and is not immediately due to an external injury. Diseases are typically linked to specific signs and symptoms and can arise from external sources such as pathogens or from internal problems such as immune system dysfunction. In humans the term is often used more broadly to cover any condition that causes pain, distress, disability or social problems, which can blur the line between disease, injury, disorder and syndrome. Diseases are commonly grouped into infectious, deficiency, hereditary and physiological categories and can also be classed as communicable or non-communicable, and the deadliest diseases worldwide include ischemic heart disease, stroke and chronic obstructive pulmonary disease.