My vitamin d tested way low after I had a second non cancerous tumor in my endocrine system.  Simply taking 5000 IU I think it is of D3 has really changed my susceptibility to head colds and such.  I guess a lot of us are deficient on vitamin D which is really turned into a hormone that supports immune response.

Cutting down on sugar helps, or really the opposite, eating too much sugar seems to hurt the immune response.

I find using a nasal rinse with the salt and the baking soda to change the pH helps shorten cold duration, or clean out sinuses during bean harvest, and I often chew a few tums then also on the theory that pH modification makes the virus environment for replication less favorable, based on apocryphal info from my chiropractor.  That is not researched medical certainty on my part.  I use the NeilMed neti pot and rinse packets with distilled water.

I personally am very susceptible to having my ears exposed in cold blowing weather.  Don’t know why, just seems to lead to instant Eustachian tube and sinus/cold issues.