If you're injured, or have flu or gastroenteritis, don't train until you are fully recovered. Although the worst flu symptoms are usually over after five days, it can take much longer to recover completely.
When you're better, start training gently and build up gradually. Don't try to make up for lost mileage - this may cause more damage or illness. Consider carefully whether or not you can run the race if you have been ill or injured.
Fit to race?
Don't run a race if you feel unwell or have been ill recently. Running may put you at risk of serious illness if you have a fever, have been vomiting, have had severe diarrhoea or any chest pains, or otherwise feel unwell.