You might not know have any symptoms, you might just have a latent source of the microorganism somewhere in your body.

你可能没有任何症状,微生物可能只是在体内某处,暗中不断滋生

耶鲁公开课 - 生物医学工程探索课程节选

Gravity is one force that acts on a body without the source of the force actually touching it.

引力是一种受力物体收到了力的作用,但却没有接触施力物体的力

耶鲁公开课 - 基础物理课程节选