3 and a half year old children with low fever and headache may be due to upper respiratory tract infection, rhinitis, encephalitis and other causes. 1. Upper respiratory tract infection. Upper respiratory tract infection is caused by mycoplasma, virus or bacterial infection, which results in low fever and headache. 2. Rhinitis. Rhinitis and sinusitis are caused by upper respiratory tract infections combined with bacterial infections, resulting in low fever and headache in children. 3. Encephalitis. Bacterial or viral infection of the central nervous system, resulting in encephalitis or meningitis, leading to low fever, headache, nausea, vomiting and other symptoms. There may also be other causes of low fever and headache in 3 and a half year old children, it is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, and then give targeted treatment or treatment under the guidance of the doctor.