3-year-old children always have fever may be related to the imperfect development of the immune system, nutritional deficiency, AIDS and so on. 1. Imperfect development of the immune system: for example, the B lymphocytes that secrete immunoglobulin have to gradually reach the adult level. Immunoglobulin is an important substance for the body to produce immunity. 3-year-old children’s immunoglobulin secretion is not as good as that of adults, so their resistance is low and they may often have fever. 2. Nutritional deficiencies: such as iron, zinc, vitamin A and other deficiencies, or protein-calorie nutritional deficiencies, etc., can easily cause abnormalities in the immune system, resulting in poor resistance in 3-year-olds, leading to frequent infections and fever. 3. AIDS: mostly due to mother-to-child transmission, the child’s immune system is defective, weak resistance to invasion of pathogenic bacteria, leading to frequent infections and fever. 3-year-old children always have a fever can also be seen in other causes, it is recommended that timely medical attention, follow the doctor’s instructions for relevant examinations, in the doctor’s help to clarify the cause, and targeted treatment or treatment.