18,000 + participants had their blood levels of "D" measured. About 3,600 study participants developed a cold during the study. Those with the lowest blood levels of "D" had the greatest risk of catching cold compared to those who had the highest levels of "D".

How much "D" is enough? Depends on many factors such as where you live, sun exposure, diet and use of sun-blockers, to name a few. A commonly accepted daily intake is around 400-600 iu/day assuming you are reasonably healthy. Extremely low levels of blood "D" can require up to 10's of thousands iu/day for a few weeks. Remember, "D" can be toxic if too high. 
 

As a precaution, ask your health care practitioner to order a baseline vitamin D on you as part of your annual check up. If you suffer from low immunity and seem to catch a lot of colds you may want to ask sooner than later.