Scanning electron micrograph of Mycobacterium tuberculosis, a species of pathogenic bacteria that cause tuberculosis A<unk>  is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury.<unk> s are often known to be medical conditions that are associated with specific signs and symptoms. A<unk>  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<unk> s, including various forms of immunodeficiency, hypersensitivity, allergies and autoimmune<unk> orders. In humans,<unk>  is often used more broadly to refer to any condition that causes pain, dysfunction,<unk> tress, social problems, or death to the person afflicted, or similar problems for those in contact with the person. In this broader sense, it sometimes includes injuries,<unk> abilities,<unk> orders, syndromes, infections, isolated symptoms, deviant behaviors, and atypical variations of structure and function, while in other contexts and for other purposes these may be considered<unk> tinguishable categories.<unk> s can affect people not only physically, but also mentally, as contracting and living with a<unk>  can alter the affected person's perspective on life. Death due to<unk>  is called death by natural causes. There are four main types of<unk> : infectious<unk> s, deficiency<unk> s, hereditary<unk> s (including both genetic<unk> s and non-genetic hereditary<unk> s), and physiological<unk> s.<unk> s can also be classified in other ways, such as communicable versus non-communicable<unk> s. The deadliest<unk> s in humans are coronary artery<unk>  (blood flow obstruction), followed by cerebrovascular<unk>  and lower respiratory infections. In developed countries, the<unk> s that cause the most sickness overall are neuropsychiatric conditions, such as depression and anxiety. The study of<unk>  is called pathology, which includes the study of etiology, or cause.
Disease
Precancerous condition
Cancer
Psychopathology
Drug
Drug 
Cancer cluster
Suboptimal health