Whether or not his theory is correct, this much I do know:
I don't take cold or flu products when I feel the early symptoms of cold/flu. I suffer for a day or so, preferrably under a blanket to sweat it out. They never seem to get too bad when I take this route. Of course, over the years, I rarely get sick - during the periods when I keep up my regular exercise routine. In the past when I have been less than diligent and slacked off on the exercise, I was more susceptible to whatever bug was "going around". Regular vigorous exercise raises your body temperature - probably tough for those little viral devils to multiply when you're getting "hot" on a regular basis.