Infections caused by the intracellular pathogen Mycobacterium avium subsp. paratuberculosis have a devastating impact on the dairy industry. Dairy cows generally are infected as neonates through the ingestion of water or feed contaminated with the bacteria. Animals will remain in the subclinical, or asymptomatic state, of the disease until a period of extreme stress, such as parturition. The clinical stage of the disease is characterized by severe weight loss, intermittent diarrhea, and fecal shedding of the bacteria. Currently, limited data exist on the effect of periparturient immunosuppression on the progression of Johne's disease to the clinical stages. Furthermore, a better understanding of the host immune response during this critical time period is essential for management of this and other diseases. The experiments described in this dissertation are guided by the hypothesis that the host immune response to MAP infection is altered during the periparturient period.