The cool thing about vitamin D and why it's the best thing you can do for the flu is that it doesn't just make your immune system stronger, it makes it smarter. When people die of influenza, they usually die because of a cytokine storm which is your immune response overreacting, causing your upper respiratory tract to become restricted and you suffocate. Adequate levels of Vitamin D prevent your immune system from this overreaction. That's how I understand it.
