At least two populations of microorganisms are found in skin microbiota: a resident flora and a transient flora. Colonization and invasion by pathogenous microorganisms is counteracted both by the host defenses and by the resident flora. Most skin infections are therefore self-limiting in healthy subjects and are defined as primary infections. Secondary infections develop on preexisting skin lesions and are usually polymicrobial and caused by microorganisms that in themselves have little pathogenic power. When immune defenses are low, secondary infections arise readily and develop rapidly. This article describes the main bacterial and viral skin diseases.