Ash, sorry to hear you feel down or sick a lot. That doesn't sound good. I think it is an interesting topic to discuss because on paper, it looks like vegan should be sick less frequently. Vegans tend to eat the foods that keep the immune system strong. My my sister who is vegan tends to get colds pretty easily. I'm sure it's just an individual thing, because we can't seem to find any trend except for the report that vegans tend to get sick less than non-vegans based on people I've talked to.
I think a lot of it is genetic. And how would we ever know if person X (a vegan) would get sick more often or less often if s/he were not a vegan? Or if person Y (a non-vegan) would get sick more of less often if s/he were a vegan?
Many non-vegans, though, (even if they're vegetarian--see the book "Vegan Freak") seem to be threatened by veganism. Strange, isn't it, that a group of primarily peaceful folks can be seen as so threatening without even doing anything?