A method of diagnosing or monitoring Mycobacterium avium subspecies paratuberculosis (MAP) infection, which method comprises detecting the presence of the polypeptide (MAP P900) encoded by the positive strand of IS900, or a fragment thereof, in a sample from a subject, wherein MAP P900, or a fragment thereof, is detected using an antibody, or an antigen-binding fragment thereof, that binds to MAP P900.