Make sure you're not eating or drinking anything that's absorbing calcium (Sometimes certain birth control can). If you live in a city, I would consider moving to a rural area (You get sick a lot less).
People over do it with lots of pills, all multivitamins are bad. If you need something specific find the vitamin from a plant extract or something of that matter, lik I take coral calcium, which has calcium, magnesium and vit. D but it's all from powdered natural coral.. Not synthasized
You've already gotten a lot of good advice - but it seems like you need a natural immune booster. Green tea can help, Echinacea, St. John's Wort and Comfrey all help with your immune system. You need to start drinking tea, or more specifically infusions of herbs that will boost your body. Peace, ~ FlowerMama