This first nationwide prospective study aims first at identifying defects within innate immunity that increase susceptibility to sepsis in children and that predispose to severe or lethal sepsis, and second at assessing the epidemiology of pediatric sepsis in Switzerland. The study started in September 2011 and is ongoing in all Swiss pediatric A clinics. This prospective multicenter observational cohort study is comparing sepsis cases with controls. Newborns and children < 17 years of age with blood-culture positive sepsis are included, thereby only assessing patients with proven infection. One blood sample will be drawn (DNA, RNA) to eventually determine SNPs in innate immunity candidate genes, blood culture isolates will be stored and in addition, a defined set of clinical parameters will be registered in order to assess demographical data, sepsis risk factors, and sepsis severity. This sampling will permit to study novel candidate genes. Finally gene-disease-association findings will be completed by studying RNA expression and protein levels. The data will be compared to a cohort of 500 healthy infants followed up until the age of 5 years who have never developed invasive infections. Severe sepsis still causes a high burden on childhood morbidity and mortality. This project will improve the current understanding of sepsis and may thus aid in developing novel strategies for diagnosis and treatment of sepsis in children.