Echinacea stimulates one side of your immune system called the Th1 response. This is the side that is responsible for killing virus and bacteria invaders. You do NOT want to overload your Th1 response on an ongoing basis as you may CAUSE an autoimmune condition, leaving you much worse that before. Contrary to popular belief, one does not 'build up' their immune system by taking immune stimulants like this. You should use these (echinacea, Vitamin C, Garlic capsules, etc) either when you feel the start of a cold or sickness, or in cycles (like 2 weeks per month).
One's immune system is improved (built up) through use. That means it gets stronger when both sides (Th1 and Th2) fire to rid the body of a bug. Other nutrients like pycnoginol and resveritol stimulate the Th2 response.
Long story short: Use your immune stimulants AS NEEDED, not all the time!
Hope this helps.