B vits are critical for metabolism - involved in protein metabolism so formation of critical immune factors etc. - the issue with b vits is the competition they have, so if you load up on one then you may be causing depletion in others. I find a high need for b2 and b6 but as jazzy said, b12 and folate are critical as well. So the opponents say that you feed the infection, but you weaken the body at the same time by not giving critical co-factors. I say their theory is bunk.