There are many causes of fever in babies, as follows: 1. The most common cause is infection. Fever caused by infectious factors can be divided into respiratory tract infection, gastrointestinal tract infection and urinary tract infection according to the site of infection, and each system infection will be accompanied by its corresponding symptoms, such as respiratory tract infection with sore throat, runny nose, coughing, coughing and difficulty in breathing; gastrointestinal tract infection accompanied by nausea, vomiting, abdominal pain, diarrhea, etc. Urinary tract infections are accompanied by frequent urination, urgency and painful urination. Infectious fever is usually caused by bacterial and viral infections, and mycoplasma infections have also increased significantly in recent years.2. In addition to infectious fever, there are also non-infectious factors, including connective tissue diseases, such as juvenile rheumatoid and lupus erythematosus, and blood system diseases, such as leukemia. There are many causes of fever, and parents need to closely observe the related accompanying symptoms. Unexplained fever needs to be promptly seen at the hospital to clarify the cause and treat according to different etiologies.