The intestinal tract is particularly prone to microbial penetration and toxin uptake, due to the large surface area, the level of nutrients and high absorptive capacity. However, it is also equipped with an array of defence mechanisms (O’Hara & Shanahan 2006). The small intestine is the site of major absorption, but is also perturbed by many intestinal infections, leading to acute diarrhea and dehydration. For example, the cholera toxin (Figure 4.3) inhibits sodium uptake, and stimulates chloride secretion. This results in fluid and electrolyte loss.