Sunshine, and supplements [700IU recommended daily allowance]
I'm taking a 1000IU D3, along with 800IU in a multivitamin. Some in regular food sources - milk has about 70-100IUs per 8oz glass.
There's an upper level - 4000IUs is too much [vitamin D toxicity after a while].
See my comment #74. My Special Forces son is now taking 5,000 units of D daily and it works for his SAD winter depression. I think this may be the newest dose recommendation.
I also wanted to mention what I would do for my children when they had a sore throat, cough, or ear infection. Starting when they were about a year old, I would make a bottle or give a cup of water with 500 mg. of crushed vitamin C, a tablespoon of honey, and 1/4 to 1/2 teaspoon of garlic power mixed in. When very young they would wake at night, start to cry, but then suckle the bottle a little and go back to sleep. I also used it when I had a dry scratchy cough and it really helped. Of course, I was also using more C in tablet form. Slippery elm losenges are natural and good.