Immunological and genetic investigations are not need when the child is suffering from undifferentiated common viral infections affecting the upper airways and when there is no family history of primary lung diseases or hereditary immunodeficiencies. The decision to perform tests should be based not only on the number of infections, but expecially on their severity, on the presence of unusual or opportunistic germs, on the protracted course and on the occurrence of infections beyond the age of primary socialization. Complete blood cell count and the dosage of immunoglobulins are considered first level tests, together with the sweat test in patients with recurrence of ear infections, bacterial sinusitis, bronchopneumonia or other invasive infections.