The identification of a new coronavirus as the etiological agent of severe acute respiratory syndrome (SARS) has evoked much new interest in the molecular biology and pathogenesis of coronaviruses. This review summarizes present knowledge on coronavirus molecular biology and pathogenesis with particular emphasis on mouse hepatitis virus (MHV). MHV, a member of coronavirus group 2, is a natural pathogen of the mouse; MHV infection of the mouse is considered one of the best models for the study of demyelinating disease, such as multiple sclerosis, in humans. As a result of the SARS epidemic, coronaviruses can now be considered as emerging pathogens. Future research on SARS needs to be based on all the knowledge that coronavirologists have generated over more than 30 years of research.