If you get bacteria under your skin, if you get a cut and somehow that cut gets - bacteria gets inside there, the bacteria can reproduce on their own.

如果你皮肤里有细菌,而你又割伤了自己,并让细菌进入了伤口,细菌自身就可以繁殖

耶鲁公开课 - 生物医学工程探索课程节选