I agree but in one case I defeated a flu that I could tell would have kept me down for a week or two. I have a history with respiratory illnesses and recognized this as a flu with a slow start, so while it was in the early lung butter phase before major fever began I committed to cardio. The goal was twofold: to exercise my lungs and dislodge mucus and ramp my temperature up, I suspect the hours of post exercise metabolic boost also helped my immune system. I started slow so as not to pass out in the case I had underestimated the infection. I took some guaifenesin (a small dose of a liquid expectorant) and walked the road for a bit, hacked up some mucous and upped the pace, I repeated this process for three days and on the fourth awoke with no lung butter. I don't normally espouse jogging but I did a lot if cardio at that pace on those 3 days, days 2 and 3 I ran, and day 3 I did sprints. I don't recommend a hard anaerobic exercise while fighting an illness, especially a respiratory one.