The causes of fever in three-month-old babies include respiratory tract infections, intestinal infections and so on. 1. Respiratory tract infection: when pathogenic bacteria enter the baby’s body, it will lead to dysfunction of the thermoregulatory center, and then there is a situation in which heat production is greater than heat dissipation, showing the symptoms of fever. 2. Intestinal infections: at the onset of the disease, the intestinal mucosa will be subject to certain damage, so that the intestinal pathogenic bacteria multiply, resulting in the phenomenon of fever; and secondly, during this period, the immune response of the autoimmune system, there will also be a fever. Three-month-old babies’ immunity is weakened and easily affected by the external environment; there are many reasons for fever, in order to avoid affecting growth and development, you should seek medical examination and treatment in a timely manner, do not operate blindly.