Disease

A disease is a particular abnormal condition that adversely affects the structure or <a href="/facts/Function_(biology)/a5hLtnS7">function</a> of all or part of an organism and is not immediately due to any external <a href="/facts/Injury/PJ7CbnMQ">injury</a>. Diseases are often known to be medical conditions that are associated with specific <a href="/facts/Signs_and_symptoms/BzP6MrTN">signs and symptoms</a>. A disease may be caused by external factors such as <a href="/facts/Pathogen/axMXtV1M">pathogens</a> or by internal dysfunctions. For example, internal dysfunctions of the <a href="/facts/Immune_system/HRCTf1pb">immune system</a> can produce a variety of different diseases, including various forms of <a href="/facts/Immunodeficiency/CIEujXmc">immunodeficiency</a>, <a href="/facts/Hypersensitivity/j2AWmqCD">hypersensitivity</a>, <a href="/facts/Allergy/dYAVosh8">allergies</a>, and <a href="/facts/Autoimmune_disorders/sojBeQ7t">autoimmune disorders</a>.
In humans, disease is often used more broadly to refer to any condition that causes <a href="/facts/Pain/70E79oUn">pain</a>, <a href="/facts/Abnormality_(behavior)/TFpmvhMk">dysfunction</a>, <a href="/facts/Distress_(medicine)/unfYpr3o">distress</a>, <a href="/facts/Social_problems/JLBMz1sm">social problems</a>, or <a href="/facts/Death/VJmq29qm">death</a> to the person affected, or similar problems for those in contact with the person. In this broader sense, it sometimes includes <a href="/facts/Injury_in_humans/PJ7CbnMQ">injuries</a>, <a href="/facts/Disability/vr9BPUxo">disabilities</a>, <a href="/facts/Disorder_(medicine)/55eeQlGL">disorders</a>, <a href="/facts/Syndrome/JFVb3x8f">syndromes</a>, <a href="/facts/Infection/182XrncP">infections</a>, isolated symptoms, deviant <a href="/facts/Behavior/0Y1VqmVX">behaviors</a>, and atypical <a href="/facts/Human_variability/B2EaDc4n">variations</a> 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 mentally, as contracting and living with a disease can alter the affected person's perspective on life.
Death due to disease is called <a href="/facts/Death_by_natural_causes/DmSBUhXm">death by natural causes</a>. There are four main types of disease: infectious diseases, <a href="/facts/Deficiency_disease/KasQy5MR">deficiency diseases</a>, <a href="/facts/Hereditary_diseases/hFqt2NLn">hereditary diseases</a> (including both genetic and <a href="/facts/Non-Mendelian_inheritance/IY6qf8hr">non-genetic hereditary diseases</a>), and physiological diseases. Diseases can also be classified in other ways, such as <a href="/facts/Transmission_(medicine)/sSGEGhcd">communicable</a> versus <a href="/facts/Non-communicable_disease/a3HxqDL3">non-communicable</a> diseases. The deadliest diseases in humans are <a href="/facts/Coronary_artery_disease/szCt0nVx">coronary artery disease</a> (blood flow obstruction), followed by <a href="/facts/Cerebrovascular_disease/Y11DmQk4">cerebrovascular disease</a> and <a href="/facts/Lower_respiratory_infections/YPY5vY6p">lower respiratory infections</a>. In developed countries, the diseases that cause the most sickness overall are <a href="/facts/Neuropsychiatric_conditions/EPNjFtFf">neuropsychiatric conditions</a>, such as <a href="/facts/Major_depressive_disorder/l2lQeWqr">depression</a> and <a href="/facts/Anxiety_disorder/wh9zFH1V">anxiety</a>.
<a href="/facts/Pathology/U9b18LJG">Pathology</a>, the study of disease, includes <a href="/facts/Etiology_(medicine)/lXMoVmyh">etiology</a>, or the study of cause.

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Disease