疾病是生物在一定原因的损害性作用下，因自稳调节紊乱而发生的异常生命活动过程，是特定的异常病理情形，而且会影响生物体的部份或是所有器官。一般会解释为「身体病况」（medical condition），而且伴随着特定的症状及医学征象。机体对病因所引起的损害发生一系列抗损害反应；自稳调节的紊乱，损害和抗损害反应，表现为疾病过程中各种复杂的机能、代谢和形态结构的异常变化，而这些变化又可使机体各器官系统之间以及机体与外界环境之间的协调关系发生障碍，从而引起各种症状、体征和行为异常，特别是对环境适应能力和体力减弱甚至丧失。

A disease is a particular abnormal condition, a disorder of a structure or function, that affects part or all of an organism. The causal study of disease is called pathology. Disease is often construed as a medical condition associated with specific symptoms and signs. It may be caused by factors originally from an external source, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune diseases. Diseases usually affect people not only physically, but also emotionally, as contracting and living with a disease can alter one's perspective on life, and one's personality.