You should definitely get your Vitamin D status up. It’s good to take vitamin K2 with that too. They really pay off with all the great effects they have on the body. Vitamin C like Able said is a must for any infection, and NAC (for glutathione) is a strong antioxidant too. Iodine and selenium are common deficiencies and help support immunity as well as thyroid function and heavy metal detox. I was just reading this article about iodine actually that says ‘Even moderate deficiency, especially in pregnant women and infants, lowers intelligence by 10 to 15 I.Q. points.’ You basically just want stuff that’s hard to get from food if your diet is balanced.
Can you find any real cranberries where you are? They’re only like 4% sugar, and it doesn’t sound like you have fructose malabsorption issues. Berry fibres tend to be prebiotic and they contain natural antifungals that fight off mould infections. You could certainly eat worse foods on the diet, but extracts are probably more reliable and convenient.