Disease is a disturbance of normal bodily function in an organism. Medicine and surgery are concerned with the recognition or diagnosis of disease and the institution of treatment aimed at its cure. Disease is usually brought to attention by symptoms, in which a person becomes aware of some abnormality of, or change in, bodily functions. Pain, headache, fever, cough, shortness of breath, dyspepsia, constipation, diarrhea, loss of blood, lumps, paralysis, numbness, and loss of consciousness are common examples.


Diagnosis is made on the basis of symptoms, signs on physical examination and laboratory and X-ray investigation; the functional disorder is analyzed and possible causes are examined. Causes of physical disease in humans are legion, but certain categories are recognized: trauma, congenital, infectious, inflammatory, vascular, tumor, degenerative, deficiency, poison, metabolic, occupational, and iatrogenic diseases.


Trauma to the body may cause skin lacerations and bone fractures as well as disorders specific to the organ involved (e.g., concussion). Congenital diseases include hereditary conditions (i.e., those passed on genetically) and diseases beginning in the fetus, such as those due to drugs or maternal infection in pregnancy. Infectious diseases include viral diseases, bacterial diseases, and parasitic diseases, which may be acute or chronic and are usually communicable. Insects, animals, and human carriers may be important in their spread and epidemics may occur. Inflammation is often the result of infection, but inflammatory disease can also result from disordered immunity and other causes. In vascular diseases, organs become diseased secondary to disease in their blood supply, such as atherosclerosis, aneurysm, thrombosis, and embolism.


Tumors, including benign growths, cancer, and lymphoma are diseases in which abnormal growth of a structure occurs and leads to a lump, pressure on or spread to other organs and distant effects such as emaciation, hormone production, and neuritis. In degenerative disease, death or premature ageing in parts of an organ or system lead to a gradual impairment of function. Deficiency diseases result from inadequate intake of nutrients, vitamins, minerals, calcium, iron, and trace substances; disorders of their fine control and that of hormones leads to metabolic disease.


Poisoning is the toxic action of chemicals on body systems, some of which may be particularly sensitive to a given poison. An increasingly recognized side-effect of industrialization is the occurrence of occupational diseases, in which chemicals, dusts, or molds encountered at work cause disease – especially pneumoconiosis and other lung disease, and certain cancers. Iatrogenic disease is disease produced by the intervention of doctors, in an attempt to treat or prevent some other disease. The altered anatomy of diseased structures is described as pathological.


Psychiatric disease, including psychoses (schizophrenia and depression) and neuroses, are functional disturbances of the brain, in which structural abnormalities are not recognizable; they may represent subtle disturbances of brain metabolism.


Treatment of disease by surgery or drugs is usual, but success is variable; a number of conditions are so benign that symptoms may be suppressed until they have run their natural course.