Impetigo is a bacterial skin infection, which manifests as small red pimples that become non-healing open sores. They may scab over with yellow or brown crusts. It's caused by a staphylococcal skin infection but if lesions are found around the nose or mouth, it may be streptococcal. Bacteria cause the initial infection, which enters small cuts or breaks in the skin. This is a very contagious disease and it can spread easily to classmates and family members if proper precautions are not taken.