1. The presence of masked swine influenza virus has been demonstrated in lungworm larvae from earthworms dug on Midwestern farms.
2. Swine influenza virus infections were provoked in 2 of 3 swine obtained from an Iowa farm during an interepizootic period.
3. The evidence presented has been interpreted as indicating that the swine lungworm is the reservoir and intermediate host for swine influenza virus in the field.
4. A concept of the epidemiology of swine influenza in which the causative virus is represented as being maintained and disseminated in a masked form by its lungworm intermediate host has been presented.