The classic primary immunodeficiencies confer predisposition to multiple infectious diseases. However since ten years severe pediatric infections which were idiopathic have now molecular explanation. Indeed, defects in several genes confer a predisposition to infection with specific pathogenes in otherwise healthy individuals. These children present a new kind of hereditary immunodeficiency with severe and/or recurrent infections caused by only one microorganisms family, in opposition of others patients with "classic" primary immunodeficiency.