Our expert says:
It's dangerous because a viral infection can very easily move to the heart muscle when you do strenuous activity (any stress can cause it and exercise counts as a stress). When this happens, you develop something called myocarditis, an infection of the heart muscle, which is easily fatal. Many runners have died as a result of this condition. So when you have flu, it's extremely dangerous to train.
The best guide to use when ill is the neck test - if your symptoms are above the neck (stuffy nose, headaches, a bit of congestion, slightly sore throat), then it's USUALLY okay to do a bit of exercise, but at a substantially reduced intensity and duration. In otherwords, if your training programme says to go and do 60 minutes of hard running followed by gym, forget that and rather do 20 minutes of running and no gym!
If your symptoms are below the neck - sore joints, aching muscles, general fatigue, chest hurting, then under no circumstances should you exercise at all. It could be very very risky, and it's not worth that. So in that case, stay in and spend your energy fighting off the flu.
Having said all this, I'm still a big believer that you should always rest if sick, even if it feels like a minor illness and you pass the 'neck test'. The reason is that your being sick is another stress on the body - you need to deal with it, not add exercise to double that stress. And so even though it may be safe to exercise when sick under some circumstances, I'm not convinced that it's the best in the long term, simply because it will delay your recovery to 100% in many cases. So two days off now may end up saving a forced one week later, if you follow my logic. Of course, this is easier said than done, but that's what I would TRY to do!
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