Histophilus somni (formerly Haemophilus somnus) is a gram-negative, nonmotile, nonsporeforming, nonencapsulated, pleomorphic coccobacillus. Pathogenic and nonpathogenic strains have been differentiated. The virulence of the organism may vary by region and age group. This bacterium can cause an acute, often fatal, septicemic disease that can involve the respiratory, cardiovascular, musculoskeletal, or nervous systems, either singly or together. The reproductive system is often affected but usually without clinical signs and without other systemic involvement.