The horse’s gastro-intestinal tract is a complex and amazing organ. It performs many functions which include processing and absorbing all the fluids and nutrients that sustain the body, as well as playing a huge role in the horse’s immune system. One of the most important functions of the GI tract is to act as a barrier between the external and internal environment—keeping the “good” stuff inside the body and the “bad bugs” out. Even though feed and water ingested by the horse may contain harmful pathogens, the barrier provided by the GI tract is usually sufficient to keep them from getting any farther, and they pass on out.