Interesting topic. I've had candida for more years than I can count, and I havent had a common cold or flu in about 14 years. I kind of attributed it to the fact that I am so obsessive about clean diet, regular exercise, and immune building supplements. I do things like paint my skin with a drop of Lugol'sIodine a couple days a week, and take trace minerals, olive leaf extract and coconut oil, which I believe can go a long way toward preventing sickness. The basic candida diet, without all the processed crap and excessive carbs, is pretty much ideal for sickness prevention as well.
But I dont have an answer to your question regarding candida recovery leading to a more relaxed immune system possibly making you more susceptible to flu etc.
Personally I would expect (guessing) that the immune system would be even stronger in fighting off common illness once the added burden of candida/heavy metals has been eliminated.