This study was undertaken to determine the extent and severity of respiratory disease caused by CAR bacillus in immunodeficient mouse strains. Mice will be inoculated intranasally with a standard number of CAR bacillus. These mice will be killed at two week intervals. Tissue will be obtained to determine the extent and severity of lesions by histopathological examination. Trachea and lungs will be cultured for presence of bacteria. The pathogenesis study is in progress and the tracheal washes confirm infection with CAR bacillus in all mice inoculated with the agent. Collected tissues are being processed for histologic evaluation. Transmission studies are being conducted and data collected.