If your babies fever is not at 100 F then they are likely fine and don't need to go to the emergency room.
If you cannot get the babies temperature to go back down to normal then you should take the baby to the emergency room because the fever could cause brain damage to the baby if it is not controlled or doesn't go back to normal.
If the babies temperatures gets to 104 F or above then that is dangerous fever temperature for a baby and the baby needs to go to the emergency room for medical treatment.
But if the fever gets to 104 F and then returns to normal within the day then they are likely okay.
But I would take the baby to the emergency room if the babies temperature ever got above 100 F just to be safe because fevers and the flu affect babies in a more dangerous way than it does adults.