This is a great question, and one I'm thinking of right now, since I just got over a really bad cold! We all want to "bring it" all the time, but in reality there are times our body needs a little bit of rest. I am not surprised I just got sick, I was running rampant with special projects at my job, still spending a bunch of time Beachbody coaching, trying to workout, and not sleeping. I hit a road block, and in a way, my body actually needed the additional recovery! (I was due for a "recovery" week anyway). So read on, I have a few suggestions for when it is and isn't ok to workout when you are sick, and then some general tips on how to get back on track with your p90x, or other routine!
A Rule of Thumb
The first rule of thumb is identify where the infection is. If it's above your neck, it is usually ok to workout, albeit at a slightly less intense level. This would be a mild head cold, sore throat, post nasal drip, etc. Definitely still do your best and forget the rest, but not at the risk of overdoing it. With a weakened immune system, the last thing you want to do is breakdown your body any more and allow for a secondary infection to come in.
If the infection is below your neck, including any heavy chest congestion or a stomach bug you should probably sit it out. I know I get to the point where I'm thinking "Oh no, if I don't workout every day I'm going to lose my results!" Realistically, it takes over a week of sedentary living to start making an impact on your progress. And I hate to say this, but usually when we are sick, we eat less, so you might lose some muscle tone with illness, but probably won't gain a lot of weight. As a health enthusiast, I cringe when I hear the phrase "I'm one stomach bug away from my ideal weight!" but, hey, we'd be lying to ourselves if we didn't realize that our appetites are usually lessened with illness.
Red Flag: Fever
One important rule is NEVER workout if you have a fever above 101. When you workout, you actually increase your internal temperature slightly, so you wouldn't want to mess with that when your internal temperature is already high!
Prevention and Getting Better
Realistically, if you are working out and eating right, and getting enough daily vitamins and nutrients, you probably won't get sick as much anyway. In fact this past week was the first time I got sick all year, I almost made it out of the winter, and considering I work with people all day that is a miracle! As for getting better, make sure you drink lots of fluids, and though I joked about appetite, you really need to eat. Your body is working hard fighting off the infection, don't starve it!
Readjusting Your Workout Schedule
If you are doing p90x or something that has a day-by-day schedule, being sick for a couple days can really mess with your head, but in reality don't sweat it. P90x is usually never JUST 90 days, I joke it often becomes p97x anyway, we all usually mess up one week! In general if you just miss one day, bump everything down a day and pretend your "missed" day was your rest day. Then you can either continue with the NEW day as your rest day, or just keep working out on your previous schedule. For example - let's say I miss Yoga on Thursday. Do Yoga on Friday, bump everything back a day and either make Thursday your new rest day, or if you like keeping Sunday as your rest day, keep on going straight through and take your next rest day on the following Sunday. Yes you are working out more consecutive days without rest, but it's only about a half a week more than usual.
If you miss more than day in a week, it's usually best to "fake" the rest of the week (pick workouts you think make sense to finish out the week) and then go back and repeat the week. There is a lot to be said in p90x and other programs about the sequence and having a "complete" week. Once you start missing strength training days, the sequence definitely gets off.
So really, if you get sick don't worry. It's important to listen to your body and do the right thing. You won't lose all your gains in a week of time. Get better and bring it the following week! You'll be stronger and rested and won't keep spiraling out of control from overworking yourself!