It's a widespread misconception that colds are caused by bacteria. Colds are actually triggered by a virus, which means if your physician prescribes you an antibiotic, it will be absolutely useless. However, the key to remember is that just being exposed to a cold virus does not have to mean that you'll catch a cold. If your immune system is operating at its peak, it should actually be quite easy for you to fend off the virus without ever getting sick.

If your immune system is impaired, on the other hand, it's akin to having an open-door policy for viruses; they'll easily take hold in your body. So the simple and short answer is, you catch a cold due to impairment in your immune system. There are many ways this can result, but the more common contributing factors are:  

1. Eating too much sugar and too many grains
2. Not getting enough rest
3. Using insufficient strategies to address emotional stressors in your life
4. Vitamin D deficiency, as discussed below read full article