This is one of the most common things I notice that guys have happen to them when using. They get two weeks into their cycle and they get a wicked cold or flu, especially with test compounds it seems. It is a major hamper on your gains at a time when you want to maximize every anabolic option you have. What to do???
I know a little bit about immune system support, and it is important to do at the first sign of a cold, but there are other things you can do year round which help as well. First and foremost that everyone knows is their vitamin C. Also don't take a gram at a time as the majority of it is wasted, break it up into 250 or 500 mg doses. And take at least a gram a day total, more if you want.
When I start to feel a cold or flu coming on, that's when I will take usually five grams a day. Also echinacea does work, it tastes like hell, but it works, and it's cheap. Keep your body tempature elevated, it is a natural immune system booster. One thing I also do, and it works as a stimulant to your immune system as well is boosting your core body tempature. I do this by having a very hot bath, and use a thermometer to check my temp.
The theory behind this is that when you get really sick, and feverish, this is a body defense mechanism to promote immune system defences. You body apparently ramps up production of WBC, things like macrophages, lymphocytes, Natural killer t-cells, and a whole gamut of others (according to the theory behind this). I have had my body temp up as high as 41.5 celcius, and it is very tough to do (I mean for willpower to stay in the bath when you feel so hot). I have found these simple measures very effective at fighting off colds, and flus.