A disease is a particular abnormal condition that affects part or all of an organism not caused by external force (see 'injury') and that consists of a disorder of a structure or function, usually serving as an evolutionary disadvantage. 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.
When caused by pathogens (e.g. malaria by Plasmodium ssp.), the term disease is often misleadingly used even in the scientific literature in place of its causal agent, the pathogen. This language habit can cause confusion in the communication of the cause-effect principle in epidemiology, and as such it should be strongly discouraged.
In humans, disease is often used more broadly to refer to any condition that causes pain, dysfunction, distress, 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, disabilities, disorders, 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 distinguishable categories. Diseases can affect people not only physically, but also emotionally, as contracting and living with a disease can alter the affected person's perspective on life.
Death due to disease is called death by natural causes. There are four main types of disease: infectious diseases, deficiency diseases, genetic diseases (both hereditary and non-hereditary), and physiological diseases. Diseases can also be classified as communicable and non-communicable. The deadliest diseases in humans are coronary artery disease (blood flow obstruction), followed by cerebrovascular disease and lower respiratory infections.