Impetigo: A skin infection caused by the staphylococcus or, less often, by the streptococcus bacterium. The first sign of impetigo is a patch of red, itchy skin. Pustules develop on this area, soon forming crusty, yellow-brown sores that can spread to cover entire areas of the face, arms, and other body parts. Most patients are children. Because impetigo is caused by a bacterium that is transmitted onto the skin, it is contagious and easily contracted by persons who might touch the affected person. Treatment involves the use of antibiotics.