" When you got cancer, you have to do chemo and it will make you worse at first. "
Depends on your definition of worse. You life expectation gets better from day one. The amount of days you're going to barf gets less from day one.
So day zero: No life expectation. Day 1: Good life expectation, (N-1) barfing days, Day 2: Good life expectation, (N-2) barfing days.