How long have you been taking the antibiotics now? Sometimes they take a few days to really kick in and get on top of it.
Yogurt for the thrush? Live plain yogurt, not sterile. For eating as well as the other end [!] as antibiotics will kill off the good bacteria in your gut as well as the bad. Or you could take pro-biotics.
If it is a viral infection then anti-biotics will not work, but you said it was an infection, so they should start working soon.
Vitamin D in the winter - I would recommend taking 25mg at first, so many people are low on Vitamin D. However, there is a warning that, combined with calcium, it could form kidney stones. I put that as I read the warning in the press; however, we take Vit D tablets, but not daily.
Zinc with vitamin C is good, and a general multi-vitamin tablet, as you sound as if your immune system is low.
Some vitamins are not worth swallowing, however Usana (expensive) are good and we have found Vitabiotics are good as well.