I had mono when I was 14 and was also very busy with sports, band, and friends. After that I was always a little more tired than I had been as a kid, and I got into cigarettes, drugs, and alcohol to make myself feel better. I only became REALLY sick, however, after a bout of food poisoning when I was 20. My current understanding of my illness is that it was brought about by the accumulation of stress (physical and emotional), which my system was too weak to handle. In other words, I think I was predisposed to adrenal insuffiency because of my genes, but I was also too hard on my body in terms of diet, substance abuse, late nights, and internalized emotion. The fact that the food poisoning brought on full-blown CFS was, I think, only the final shock of a slow, gradual process.