Parents.com
If your infant is 2 months or younger and has a rectal temperature of 100.4 degrees F. or higher, he could be sicker than he seems. (One exception to this rule is if your 2-month-old just got his vaccinations and runs a low-grade fever within 36 hours.) Young babies usually don't show a lot of symptoms, but they can quickly develop a serious bacterial infection because their immune system is still immature, says Sue Hubbard, MD, a pediatrician in Dallas. Parents often assume their baby just has a cold, but colds usually don't cause a fever at this age. A doctor will see an infant with a fever as soon as possible, but if it's the middle of the night, your pediatrician may send you to the ER.