There are numerous ways to boost, and maintain immunity.
Water, Sunshine, and a balanced diet. All hard to attain.
Water, drink 1/2 of your body weight per day of filtered water.
Vitamin d3 supplement on a regular basis, upto 5000 IU, as a loading dose, then down to 2000Iu daily.
A good multi vitamin containing Vitamins A,C,and zinc.
Probiotics, either in supplement form. Or increase dietary intake. They are present in "live" foods, in there natural form. As well as yogurt, cottage cheese, and other fermented foods.
Juicing is also very beneficial ways to boost immunity. Juices containing live fruits, and vegetables, contain beta glucans, that boost immunity. May also be taken in supplement form.
During times of illness, replace tooth brushes, or sanitize in the dishwasher, or soak in peroxide, as you can re infect your self.
If you find that you are sick way to often, implementing these holistic measures will over time decrease your incidence of sickness. Starting this regime before cough and cold season will go a long way in your pursuit of being well.