A disease is an abnormal condition of an organism that impairs bodily functions, associated with specific symptoms and signs.
In human beings, “disease” is often used more broadly to refer to any condition that causes discomfort, dysfunction, distress, social problems, and/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.
Classifying a condition as a disease is a social act of valuation, and may change the social status of the person with the condition (the patient). Some conditions (known as culture-bound syndromes) are only recognized as diseases within a particular culture. Sometimes the categorization of a condition as a disease is controversial within the culture.
This chart, compiled in 2002 from the global burden of disease study performed by the World Health Organization shows an overview of the impact of various classifications of disease, segregated by regions with low and high mortality:

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