Regarding one's constitution - this may have a lot to do with the immune system. For instance, if someone's immunity is quite strong, they will be less susceptible to succumbing to the local virus doing the rounds. Scientifically, this may come down to the number of white blood cells (that combat foreign invaders). A good immune system can be due to many influencing factors e.g. diet/nutrition but also the mindset. I have a hard wired positive mindset and am rarely ill. (I also take a good vit/min supplement though and eat reasonably healthy, i.e. not very processed).