I suggest you see a good naturopath. They will likely put you on some high-powered vitamins to boost your general vitality, as well as some herbs targeted specifically at boosting immune function. You should get results within a fortnight.

My aunty was telling me she had a rotten health run and started taking a product called either ArmaForce or Armaguard - purchased at her pharmacy - and three weeks later she feels back to her normal self. It too contains all those immune-boosting herbal things. HTH