Yersinia enterocolitica is one of the most common causes of food borne gastrointestinal disease. After oral uptake yersiniae replicate in the small intestine, invade Peyeŕs patches of the distal ileum and disseminate to spleen and liver. In these tissues and organs yersiniae replicate extracellularly and form exclusively monoclonal microabscesses. Only very few yersiniae invade Peyeŕs patches and establish just a very few monoclonal microabscesses. This is due to both Yersinia and host specific factors.