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.


Infectious diseases

An infectious disease is a disease caused by any microorganism, but particularly viral and bacterial diseases and parasitic diseases, in which the causative agent may be transferred from one person to another (directly or indirectly). Knowledge of the stages at which a particular disease is liable to infect others and of its route (via skin scales, cough particles, clothing, urine, feces, saliva, or by insects, particularly mosquitoes and ticks) helps physicians to limit the spread of disease in epidemics.


Bacterial diseases

A bacterial disease is a disease caused by bacteria or their products. Many bacteria have no effect and some are beneficial, while only a small number lead to disease. This may be a result of bacterial growth, the inflammation in response to it, or of toxins (e.g., tetanus, botulism, cholera). Bacteria may be contracted from the environment, other animals or humans, or from other parts of a single individual. Infection of skin and soft tissues with staphylococcus or streptococcus leads to boils, carbuncles, impetigo, cellulitis, scarlet fever, and erysipelas.


Abscess represents the localization of bacteria, while bacteremia and (typically) sepsis are conditions in which a bacterial infection is circulating in the blood. Sometimes a specific bacterium causes a specific disease (e.g., anthrax, diphtheria, typhoid), but any bacteria in some organs cause a similar disease: in lungs, pneumonia occurs; in the urinary tract, cystitis or pyelonephritis, and in the brain coverings, meningitis. Many venereal diseases are due to bacteria. In some diseases (e.g., tuberculosis, leprosy, rheumatic fever), many manifestations are due to hypersensitivity (see immunity) to the bacteria. While antibiotics have greatly reduced death and ill-health from bacteria and vaccination against specific diseases (e.g., whooping cough) has limited the number of cases, bacteria remain an important factor in disease.


Viral diseases

A viral disease is an infectious disease due to a virus. The common cold, influenza, chickenpox, measles, and rubella are common in childhood, while lassa fever and yellow fever are important tropical diseases. Viruses may also cause specific organ disease such as hepatitis, meningitis, encephalitis, myocarditis, and pericarditis. Most viral diseases are self-limited and mild, but there are few specific drugs effective in cases of severe illness. Prevention by vaccination is therefore crucial.


Parasitic diseases

A parasitic disease is an infection or infestation by parasites, usually referring to nonbacterial and non-viral agents (i.e., to protozoa and helminths). malaria, leishmaniasis, trypanosomiasis (see trypanosomes), chagas' disease, filariasis, schistosomiasis, toxoplasmosis, amebiasis, and tapeworm are common examples. Manifestations may depend on the life cycle of the parasite; animal or insect vectors are usual. Chemotherapy is often an effective treatment.



Pathology is the study of the anatomy of disease. Morbid anatomy, the dissection of bodies after death with a view to the discovering the cause of disease and the nature of manifestations is complemented and extended by histology. In addition to autopsy, biopsies and surgical specimens are examined; these provide information that may guide treatment. It has been said that pathology is to medicine what anatomy is to physiology.



A pathogen is a microbe capable of causing disease. Pathogens include many viruses, bacteria, fungi, and protozoans.