In research on influenza, little attention has been given to factors that determine the patterns of infection in human adults or infants and the severity of disease. Ferret influenza has been used to elucidate the following facets of pathogenicity that bear on these questions about human disease: the differential infectivity of virus strains for the upper respiratory tract (URT); the reasons for less severe infection of the lower respiratory tract (LRT) than of the URT; why pneumonia is rare; and why strains differ in the production of LRT infection. The origin of fever has been defined; viruses have been shown to differ in fever-producing components. Poor spread of virus from the respiratory tract to other susceptible tissues and rarity of fetal infection have been explained. Death in neonatal ferrets due to influenza with either a syndrome akin to cot death or viral pneumonia have been elucidated, and protection of the young by immunized mothers has been demonstrated.