Looks like you have gotten some very good advice. I would add that instead of ditching your mulit-vitamin you should instead consider a high quality product instead of one that is mostly just petro chemicals and the like (unless you are already taking a high quality product - such as the multi-vitamins at Dr Mercolas or Oasis Advanced Wellness). I would also seriously consider a good plant derived trace mineral supplement and perhaps iodine as well. Grapeseed extract and colustrum can help boost your immune system to help ward off illness.
Perhaps you should take a good look at your diet and see what you may be missing - as well as what your illnesses have been and the clues that might provide. Omega 3's for example. B-12 and folic acid. You may be able to make a few dietary adjustments or you may wish to consider supplementation where needed. Personally, if you aren't taking coconut oil, I highly recommend it.
Some may tell you that you can get all the vitamins, minerals and other nutrients you need from diet alone. They are badly mistaken. I am not recommending that you load up on supplements, but in today's reality of depleted soils and processed foods, even the healthiest of diets will still leave you deficient in some minerals and many trace minerals.