Losing qi to bugs

EVER since I started practising qigong nearly 15 years ago, I definitely noticed an improvement in my health, and in my energy levels. Several years later, I started taking health supplements, and am now an ardent believer and promoter of the usefulness of adequate supplements as an essential component of nutritional health and healing. 

For the last three years or so, I have not had any cough-and-cold. Every time I felt the prodromal (early) symptoms of itchy nose and throat, these would promptly go off. Previously, these symptoms would invariably develop into runny nose, sore-throat, cough and usually fever as well. And it would usually take one to two weeks for me to recover fully.