It’s not clear whether vitamin D specifically protects against H1N1, a novel virus, but there’s growing evidence that it does protect against a number of respiratory infections – and that many Americans do not get enough of the vitamin.

Yes – a blood test called 25-hydroxyvitamin D, which, though expensive ($20 to $100 a pop), is covered by insurance. Your level should be at least 30 nanograms per milliliter of blood and not more than 100. If your level is way below that, say around 18, you need to take 1,200 international units or more a day of vitamin D supplements, either the type called D2 or D3, said Dr. Michael F. Holick, an endocrinologist and leading vitamin D researcher at the Boston University School of Medicine.

Several recent studies presented at meetings of the American Association for Cancer Research and the American Society of Clinical Oncology suggest that adequate consumption of vitamin D- which most Americans do not get – is linked to lower risks of breast cancer.