What foods does it consist of?
In addition to the article that he other guy posted, consider foods that have probiotics. A healthy gut microbiome will work wonders for your immune system.
Here’s another article, also by healthline. This one focuses on probiotic foods to give you a good idea. The only thing I don’t agree with in this article is that they put yogurt above kefir, but most research seems the point towards kefir being the best probiotic. But other than that it’s a great article.
https://www.healthline.com/nutrition/11-super-healthy-probiotic-foods#TOC_TITLE_HDR_5
Also i would suggest going the sugar free route here with these foods and avoiding flavored kefir/yogurt.
Another thing; try to have some diversity in your diet. That will provide you with a good mix of different vitamins, minerals and probiotics which is ideal.
Avoid empty calories like flour and sugar. Fruit sugar isn’t that bad since it also comes with vitamins and fiber so unless you have a medical issue you don’t need to avoid fruits.