Mechanisms to provide intestinal absorption of larger molecules are very limited in the intestines—the principal path to absorption of these molecules is first metabolism—to break large molecules down to smaller entities that can then be absorbed. There are five different mechanisms by which materials can pass from the mucosal side of the gastrointestinal (GI) tract across to the serosal side: passive diffusion through a lipid membrane; diffusion through pores; active energy-dependent transport; absorption through lymphatics; and absorption of macromolecules by pinocytosis. In the neonatal period when the pig and mouse have an immature intestinal epithelium, they are more susceptible to enteric pathogens like Escherichia coli and rotavirus, and they can internalize toxins like endotoxins from gram-negative rods. The absorptive surface of the intestinal mucosa has many folds called valvulae connivents which increase the surface area of the absorptive mucosa about threefold. Water is transported through the intestinal membrane entirely by diffusion, obeying the usual laws of osmosis.