The most common causes of fever in one-year-old children are firstly considered infections, such as viral, mycoplasma, chlamydia or bacterial infections, which need to be analyzed according to the child’s symptoms and the results of blood tests. The next common causes of fever are rheumatic immune system diseases and neoplastic diseases. In addition, some congenital genetic and metabolic diseases should be considered. For fever caused by different diseases, there are usually specific accompanying symptoms, and some specific tests can be completed to help diagnose.