It is frightening to see an active child become suddenly lethargic during a viral fever or cough, but this is often a common symptom of the illness itself. If your doctor has ruled out serious concerns, focus on keeping your child hydrated with ORS and offering bland, easily digestible foods. If the fatigue is recurring or feels excessive, continue to monitor for other symptoms like fever or cough and consult your pediatrician to ensure a smooth recovery; they may suggest probiotics to help their gut health during this time.