When bodybuilders start to feel ill panic sets in, because, not only do we see it as a time where we cannot train and progress, but if the illness affects the appetite, we may actually lose size! In reality though, there will be minimal losses from a common cold, tummy bug or food poisoning. Even with a dose of influenza for a couple of weeks there may be some catabolism, but this will be easily got back when you restart training and eating properly. Don't weigh yourself as you will have lost weight, your glycogen stores will be low and much of weight loss will be water; the scales could make you depressed.
Don't panic - accept the illness and rest up. I would advise against weight training as this will use up energy and nutrients needed to fight the illness, and will be more stress on the system. If you have a cold, for the early heavy stages don't train, then go back to the gym, and train as you can manage without overdoing it. Remember to keep warm if you come out from the gym sweaty.
Some illnesses will not affect appetite, in which case, continue with your normal good diet including plenty of fluids. If your appetite is diminished, eat small and often of whatever you can - junk food is ideal at this time because it's full of valuable calories and is tasty.
The hardest situation is if you are nauseous or vomiting. During nausea, eat what you can and often fluids are tolerated fine, even if food isn't. We've all had those illnesses when we can't even stomach water despite being extremely thirsty. The advice here is don't even try; the mere thought can make you worse! Each hour just try a sip of water or suck on an ice cube, if this stays down progress firstly to a small glass of water, then to a sugary drink. The first foods tried should be toast or a plain biscuit, eventually progress to a small meal.
Don't rush back to the gym too soon! It may be better to wait an extra day rather than have further setbacks. If you continue to suffer from sickness and diarrhoea for more than 24 hours seek advice from your doctor.