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.
Although forests are dynamic systems, the speed of change to which they are subjected is now at an unprecedented level. This is due to factors such as climate change, nitrogen deposition, the introduction of invasive species, and the loss of biodiversity and habitat.
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