Disease is a specific abnormal condition that negatively affects the structure or function of all or part of a living body and is not caused by immediate external injury. Diseases are commonly recognized as medical conditions associated with particular signs and symptoms. A disease may result from external factors such as pathogens or from internal dysfunctions. For example, abnormalities within the immune system can lead to many different illnesses, including forms of immunodeficiency, hypersensitivity, allergies, and autoimmune disorders.

Among all species, the term disease is often used more broadly to describe any condition that causes pain, dysfunction, suffering, social complications, or death to the affected individual, as well as related hardship for those around them. In this wider sense, it may include injuries, disabilities, disorders, syndromes, infections, isolated symptoms, abnormal behaviors, and atypical structural or functional variations, though in other contexts these may be classified separately. Diseases may affect individuals not only physically but also mentally, as living with illness can profoundly alter one’s perception of life and the world around them.

Death caused by disease is referred to as death by natural causes. There are four major categories of disease: infectious diseases, deficiency diseases, genetic diseases (including both hereditary and non-hereditary conditions), and physiological diseases. Diseases may also be classified in other ways, such as communicable and non-communicable illnesses. In more developed societies, the illnesses responsible for the greatest burden are often neuropsychiatric conditions such as depression and anxiety.

The study of disease is known as pathology, including the investigation of causes and origins, referred to as etiology.

Despite their immense resilience and elemental power, dragons, like all other species, are not immune to disease. Any living creature can fall ill. Many plants, animals, toxins, and natural organisms pose dangers that even healing magic and Red Crystals cannot cure.