A disease is any condition which results in the disorder of a structure or function in a living organism that is not due to any external injury. The study of disease is called pathology, which includes the study of cause. Disease is often construed as a medical condition associated with specific symptoms and signs. It may be caused by external factors such as pathogens or by internal dysfunctions, particularly of the immune system, such as an immunodeficiency, or by a hypersensitivity, including allergies and autoimmunity.
Cancer risk in humans exposed to low radiation doses is usually calculated based upon estimates obtained at higher doses. As many cancers originate in stem cells, studying low dose radiation effects on these cells is particularly important.