Johne's disease (paratuberculosis), caused by Mycobacterium avium subspecies paratuberculosis (MAP), is a chronic progressive disease of cattle causing protein-losing enteropathy, intractable diarrhoea, profound emaciation, and death. The disease has a long subclinical phase of 2-5 years before clinical signs appear, during which infected animals shed bacteria and experience subclinical welfare compromise affecting immune function and production. Clinical Johne's disease causes significant suffering over weeks to months; welfare-positive management requires prompt euthanasia once clinical signs are advanced. Herd prevalence is 20-40% in many dairy industries globally. Control relies on test-and-cull programmes, hygiene management to prevent calf exposure to adult faeces, and colostrum management. No effective vaccine or treatment exists. MAP has been investigated as a potential trigger for Crohn's disease in humans (zoonotic significance uncertain). Welfare screening programmes enable early detection and culling decisions before advanced disease suffering.