It has to do with focus. When I'm sick I don't focus on "I am sick". I don't allow myself to feel sorry for myself. But think continuously about how I feel when I'm not sick. Then I drive my mind all the time through "I don't have time for this, I want to get up!" This has always worked. Through the years I've learnt to keep my mind focused on health, wellbeing and strength at all times. Therefore I am almost always energetic.