Re: Regimen For Preventing Colds/Flu...
Generally eating a lot of fruits and vegetables, getting enough exercise and getting enough sleep each night is the best way to support the immune system. Vitamin D is crucial, but unless you have blood-test confirmed deficiency go easy on supplements and try to get it from food, as overdose is possible.
However, depending on your job (schoolteacher, working in a pediatrician's office, bus driver) it may be that you are exposed to more germs than any human immune system could possibly ward off. Never mind what's "natural." Humans living in cities and riding airplanes is unnatural, and until we discovered hygiene in the 19th century, contagious disease was a constant terror to the strongest and healthiest people.
Sorry, there's no "cold prevention" formula. Handwashing is the only real solution.