Recurring colds

I start again on MFP, be good for around 2 weeks, start to see the lbs disappearing, and then I ccome down with a cold. So I up my calories 'just to help fight it,' but then fail get back on the wagon. I don't know how many times this has happend in the last year or so. I came back again the begining of January, and, you've guessed it, I'm ill. Again. Anyone else struggle with colds? I've tried vit C, and echinacea but neither seem to help, my resistance seems to plummet everytime I try to loose weight.