a. Laboratory group. This consists of biochemists and biologists experts in several molecular biology techniques applied to BALF specimens. These investigators will remain blinded to the clinical data, which will be known only to the project coordinators. b. Clinical group. This will consists of all clinicians -neonatologists and/or pediatric intensivists -working in intensive care units (at the University Hospital "A.Gemelli" or in so called "Collaborating centers") where patients' enrolment, samples and data collection will be performed.