What temperature is counted fever for infants? My baby is 2 months old. She is having a temperature of 37.8 What do i do?

as my 2 kids when they babies always gets fever and sometimes fit too as its too high.as i k ow usuall they says if its over 38 u need to quickly sponge and if it doesnt go down its best to see a doc or when its at night time A&E.