How can 3 week old baby have fever?

Infants at that age can have a fever for varying reasons. Until they are 2 months of age, a fever above 101 degrees Fahrenheit should be taken seriously. Contact your physician for treatment ASAP. This isn't something to panic about, but just to be monitored closely. Usually a treatment of infant Tylenol and infant Motrin are ordered. After 2 months of age a fever of 103 or less is not alone something to panic about. If it doesn't go down with infant Tylenol lasts longer then 24 hours, or is accompanied any other symptoms contact your child's physician.