This is the question we regular exercisers agonise over when lurgy strikes.
Those unconvinced of the joy of exercise will struggle to comprehend why it is we WANT to train though the illness - why not take the opportunity to 'pull a sickie' on our fitness regimen. Those people will always say, "Your body obviously needs a break. Rest."
Conversely, the exercise-obsessed, the training sadomasochists amongst us will insist that we should "Suck it up! Sweat it out!"
So who's right?
It depends on the circumstances and the symptoms.
If you have 'above the neck' symptoms i.e. headache, runny nose, sore throat but DON'T have a fever, then you can work out as planned, just notch the intensity down from high to moderate. As soon as these symptoms disappear, you can resume high intensity training.
If you have 'below the neck' symptoms i.e. muscle aches (non workout-related!), a chesty cough, swollen lymph glands, vomiting, diarrhoea, extreme tiredness, then either rest, or keep workout intensity low. You can increase to moderate intensity once the symptoms start to subside, but give it a good two weeks before increasing back up to high intensity workouts.
Do NOT exercise if you have a fever. A temperature indicates your body is trying to fight something, so adding stress by working out is going to inhibit this process, at best prolonging the illness and at worst, causing it to develop into something much more serious.
Do NOT work out at the gym whilst you're contagious. You might feel up to it, but the other gym users at risk of being infected will be less than appreciative. Don't be selfish - stick to home or outdoor training.
Do NOT give up altogether. Use your downtime to focus on general well-being - meditative yoga and short walks are both great options when training is out of the question.
Do NOT immediately try to make up for lost time. Once you're able to resume workouts, don't attempt to start from where you left off: instead, gradually work back up to your usual training intensity. Doing too much too soon could potentially trigger another bout of illness - your immune system will be weakened and you'll be extra susceptible. Apply common sense and listen to your body. If you start to feel dizzy or nauseous, slow down.
Finally, if you're unsure of your symptoms and what you can or can't do, consult your GP. Better safe than sorry!