So you're feeling under the weather, and now you can't work out, and you're worried about staffing on track...well, I have good news for you! I don't claim to be a doctor or a specialist of any kind, but research and personal experience have led me to believe that sometimes working our when your sick keeps your immune system up and keeps you physically healthy which can only help you get better.
HOWEVER, you need to CUT BACK, at least a little, while you aren't feeling like your normal self. If you have body aches and feel weak or are vomiting or have a fever, then I'm going to suggest that you NOT work out and gets lots rest (sleep helps you're metabolism work better!) and just stick to drinking water. But if you just can't stay away then a short, show and EASY walk should be okay.
Now, if you just have a head cold, simply cutting back on how long or how hard you work it can be enough. For example: If you're doing Jillian Michaels Yoga Meltdown, maybe only do the first 2 circuits.and three cool down and stick with three beginner version; if you're running, cut back 15 minutes or maybe just speed walk; if you're weight lifting, maybe just stick to a little bit cardio or lower weights and reps. (You get the idea.)
So remember, just because you're sick, it doesn't mean you're going to gain weight or get off track! Just modify! :-)
Check out what CNN has to say.