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Illness and sickness are generally used as synonyms for disease.[4] However, this term is occasionally used to refer specifically to the patient's personal experience of their disease.[5][6] In this model, it is possible for a person have a disease without being ill (to have an objectively definable, but asymptomatic, medical condition), and to be ill without being diseased (such as when a person perceives a normal experience as a medical condition, or medicalizes a non-disease situation their life). Illness is often not due to infection, but to a collection of evolved responses°™sickness behavior by the body°™that helps clear infection. Such aspects of illness can include lethargy, depression, anorexia, sleepiness, hyperalgesia, and inability to concentrate