Recently the FLU is very strong. Many babies going to see the doctor has fever. And many of them fever for 4-5 days, during which there may be up and down on the temperature. So keep seeing the doctor at 2-3 days, in order to hear the lung to check any other disease.
Let the baby take more rest and drink some more water. Use ice bag or fever patch.
Check the temperature every 2-3 hours.
If fever continue to be very high at around 40"C for many hours - then better go to see doctor.