I eat a lot of fruit and veggies every day. When I'm sick I eat even more. I'm not a naturopath, and I don't think that vegetables are an alternative to, say, antibiotics. But I don't think all those vitamins do anything to hurt either.
That said, sometimes I do imagine that the cells in my immune system appreciate the vegetables that I eat, treating my meals like reinforcements to a well stocked armoury. I like to believe that cucumbers and carrots are being swung like lances through invading bacterium, chilli's hoicked into pathogen filled bunkers like grenades. The soft tops of broccoli and cauliflower doused in fish oil, lit on fire and launched like a Molotov Cocktail into a swarm of advancing germs. My wounded T-Cells returning to the field hospital in the upper levels of the healthy food pyramid in my heart.