Three and a half year old baby boy with low fever and vomiting, consider acute upper respiratory tract infection, acute gastroenteritis, viral encephalitis and other possibilities, you can take general treatment, anti-infection, anti-virus and other medication to improve. 1. Acute upper respiratory tract infection: many acute upper respiratory tract infections manifest as low fever and are combined with vomiting and gastrointestinal symptoms. Fasting within two hours after vomiting, oral rehydration salts after two hours, at the same time, low fever to take physical cooling to reduce fever; consider influenza can use oseltamivir and other antiviral treatment. 2. Acute gastroenteritis: low fever and vomiting at the onset of the disease, followed by a rise in body temperature and diarrhea. Physical cooling at the same time, can be montelukast to protect the gastrointestinal tract, vomiting severe cases need to be infused with fluids to replenish water and electrolytes. If there is bacterial infection, anti-infection treatment such as cefixime is needed. 3. Viral encephalitis: at first, it can be manifested as low fever and vomiting, which can be jet vomiting, accompanied by headache and drowsiness. The main thing is to lower the cranial pressure such as mannitol, respiratory support therapy if necessary, and at the same time also need to use acyclovir and other antiviral drugs treatment. 3 years old baby boy with low fever and vomiting, may also be appendicitis and other surgical diseases, it is recommended to go to the hospital, improve the examination to clarify the cause of the disease, to give targeted treatment or treatment. The above medication is in accordance with the doctor’s instructions.