A 45-day-old female infant is brought to the emergency department by her parents due to fever. Her mother reports a rectal temperature of 102.0°F (38.9°C) at home one hour ago. The infant has been fussier than usual for the past 12 hours. Her oral intake has slightly decreased. She is breastfeeding every 3 hours instead of her usual 2-2.5 hours. She has had 5 wet diapers in the last 24 hours, which is less than normal. There is no cough, congestion, vomiting, or diarrhea. The infant was born at 39 weeks of gestation via spontaneous vaginal delivery. There were no maternal complications. She has received no immunizations yet. There are no sick contacts at home. Her parents deny any trauma or recent travel.