A 4years old female child( BW- 15kg) has been suffering from bouts of fever,cough and cold, stomach upset and episodic vomiting almost every 2-3 weeks for the past three months. She has been treated mostly with symptomatic medicines and twice with antibiotics. Blood R/E on one occasion and urine R/E C/S on two occasions were inconclusive. What may be the cause?

How should I manage this patient? Is there any specific investigations that need to be done?