If your symptoms are below the neck such as chest congestion, intense coughing, vomiting or diarrhoea, let your illness run its course before you start running again. Running under those conditions increases dehydration and may cause more serious issues. You also should never run if you have a high fever.
Use the above/below the neck rule. If your symptoms are above the neck such as a runny nose, sneezing or sore throat, you can run. Just take it easy and don’t do any intense workouts. Be sure to be aware of any symptoms such as dizziness, nausea or profuse sweating. If these symptoms occur you should stop running.