Disease is any condition that disrupts a healthy balance between body systems and produces discomfort, dysfunction or even death for the affected individual. It can be caused by infections with pathogenic microorganisms including viruses, bacteria, fungi and protozoa, or by non-infectious agents such as multicellular organisms and aberrant proteins known as prions. In a wider sense the term is often used to describe any disorder that causes pain, distress or social problems in the affected individual or similar problems for those in contact with them. This includes injuries, disabilities, syndromes, atypical variations in structure and function, deviant behaviors and isolated symptoms. At a global level diseases are catalogued in the International Classification of Diseases (ICD), which is currently in its tenth revision. The ICD lists health problems based on a number of criteria and also includes some things that are not technically diseases, such as dehydration.
Some diseases are transmissible, spreading from person to person via the release of pathogen particles in bodily fluids. Others are not, and they may be caused by internal sources such as autoimmune or genetic dysfunction. Diseases can be either acute, which means that they produce symptoms rapidly and are expected to disappear in a short period, or chronic, which means they persist for a long time.
Signs of disease can be noticed by medical professionals, such as physicians or nurses, but they can also be picked up by untrained individuals. These signs include external indications of disease, such as a fever, rash or clubbing of the fingers, and internal indicators, such as an altered blood pressure or altered chemical composition in urine or stool. Some diseases have clear cause and source, such as infectious diseases, while others are known to have an unknown cause, and so are classified as idiopathic disease.
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