One is a human being, so when one is not healthy, one can't work at your typical 100%.
Like at school and work, when you are ill, you can do properly your activitites, so the boss or the teacher needs to be more flexible and understand it instead of being more demanding.
The physical changes are not compared, however, with how you feel at the moment: weak, drowsy, prefering to stay in bed, feeling headaches or stomaches, etc.