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.
One of the most important questions posed by climate change is whether species can respond to a changing climate quickly enough to avoid extinction. In order to survive, populations must either be able to adapt to or tolerate the change in environment, or migrate to more suitable conditions.