I have read a number of studies regarding vitamin D and virus. One study was recently reported on CNN showing that vitamin D had no effect on cold prevention. In that study, researchers gave subjects the equivalent of 3500 units/day. However, the effect of vitamin D is dependent on blood level, not dosage. Studies that compare vitamin D blood level show that there is an effect on virus protection, as well as other benefits including cancer protection.
According to doctors, the "normal" range is about 20-80 (varies a bit by source.) So, if you have a level of 22, you are normal. If taking 3500 units brings you up to 32, you are still normal, but you will likely get colds and flu. Properly done virus studies show that the optimal level is more like 50-80, and that cancer prevention attributes appear in the 70-100 range. Some references suggest that a level above 100 may be harmful, but the negative effects are suspect, as is the level at which they start. So, I suggest shooting for a number at least 60, preferably 70-90.
How? Have your level tested next time you visit the doc - this test is cheap. The studies also show that many folks need at least 8,000 units/day to achieve optimal level, so disregard the doc's advice and take 10,000 units/day if your level is under 60. Re-test in 3-6 months. If you are still low, increase the dose. If over 100, decrease. Re-test every year. You can get 360 pills of 10,000 units from drugstore.com for $20.
My story: I used to get a cold or flu virtually every season. Four times a year, almost without fail. About 6 years ago my doc tested my D and I was at 18. I started monitoring my D level, and have kept it above 60 by taking 10,000 units/day, every day. I have not had a cold or flu, or any other illness, in those 6 years. Of course, there may be other factors involved, but I am convinced vitamin D is the main reason.