Sentence:  When <WSD>homeostasis</WSD> is disrupted by an external stressor - such as injury, lack of nutrients, or invasion by parasites or other organisms - illness may occur. 
Meaning of the word marked with <WSD> tags:  The ability of a system or living organism to adjust its internal environment to maintain a state of dynamic constancy; such as the ability of warm-blooded animals to maintain a stable temperature.