Why?

Meh. This term I have made a concerted effort to be healthy. Yet, despite taking more exercise, getting to bed at a decent time, eating lots of fresh fruit and veg and drinking nothing but herbal tea, I once again find myself beginning second week surrounded by vitamin C, zinc, echinea, Kleenex balsam and an inability to concentrate cos my head's all stuffed up. What more can I do??!!!!!