In most cases, a four-month-old baby with fever has a viral infection or bacterial infection that may lead to respiratory symptoms, such as upper respiratory tract infections, which are commonly known as cold symptoms, including nasal congestion, runny nose, sore throat, and coughing or coughing up phlegm. It may also lead to lower respiratory tract infections, such as bronchitis or pneumonia. If bronchitis or pneumonia occurs, it is often more serious and the child is younger, so it is better to be hospitalized for infusion therapy for better treatment. There may also be digestive symptoms, such as fever caused by rotavirus infection or diarrhea caused by bacterial intestinal infections, which may lead to fever. Once the virus or bacteria grows and multiplies in the baby’s body, it will stimulate the body to produce a lot of toxins, which will stimulate the thermoregulatory center to move up the temperature setting point and lead to fever.