This document is designed to serve two purposes: (1) to provide guidance on health protection to employers whose employees may be exposed to avian influenza (AI) viruses; and (2) to provide technical information (in appendices) about AI viruses and, in particular, about H5N1, an AI virus currently circulating in Asia, Europe and Africa that rarely causes disease in humans but when it does the case fatality rate is high. This document updates guidance on avian flu issued by OSHA in March 2004.