It depends on how close I'd get sick to the contest and whether or not I'd have enough time to get to where I need to be on contest day. If I was cutting it close, I'd say fuck it and not compete. Or you can still compete, get your ass kicked, and use getting sick as a scapegoat? Trust me on this, 99% of the people that know you're competing want you to lose. Then you can count on everyone asking you how well you did in your show for months-even if they already know you lost your ass. The practically get off on hearing you say that you lost. Lame excuses only make them feel even better cuz it looks like you psychologically can't handle the loss yourself. I remember my novice days of competing. I thought that I was better than I was and placed poorly. I would dread going to the gym for weeks-especially when some motherfucker that was sitting in the 5th row/center at my contest approaches me and asks me how I did. I'd be like-"you were there right?-so why the fuck are you asking me if you already know you jealous pencilneck geek"
However, if you're not that serious or it's your 1st show you might be better off doing it regardless of cold related setbacks just to prepare yourself better for the next time? The experience of competing, win or lose, will definitely help for your next show. What it comes down to really is whether or not your ego can handle it if you don't do as well as you might have liked to.