How can good stress do to your body? and how does it affect you?

"Good" stress: Any kind of change is "stress" -- and the body responds to it with its own mechanisms. Some are beneficial, such as white blood cells' helping to heal wounds, plus your ability to mount a fever in response to infection. Within limits, stress can help you focus and become more mentally sharp for tasks. Out of control, stress burns you out. Think of fire in a wood stove, vs wild in dry grass.