Here's what I recommend taking every day for a strong immune system:
Vitamin D3 - 10,000 IUs
Vitamin C - 2,000-3,000 mg (preferably powdered ascorbates, not ascorbic acid)
Quercetin - 800 mg
Zinc - 25-50 mg
Probiotic - 20 billion cells
Omega 3 - 800 mg
CoQ10 - 100 mg am, 100 mg pm (gelcap not powder)