Some pathogenic bacteria live constantly in the body but cause no trouble unless there are changes in body which lower its resistance, such as local irritation, fatigue, heat or cold, or hunger or thirst.

• 有些致病菌一直生活在人体内而不致病，除非是体内发生了像局部刺激、疲劳、冷孤饥饿或干渴等变化，降低了抵抗力。