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 any external injury. Diseases are often known to be medical conditions
that are associated with specific signs and symptoms. A disease may be caused
by external factors such as pathogens or by internal dysfunctions. For
example, internal dysfunctions of the immune system can produce a variety of
different diseases, including various forms of immunodeficiency,
hypersensitivity, allergies, and autoimmune disorders.