The cytotoxin-producing organisms adhere to the mucosa, activate cytokines and stimulate the intestinal mucosa to release inflammatory mediators. Invasive organisms, which can also produce cytotoxins, invade the intestinal mucosa to induce an acute inflammatory reaction, involving the activation of cytokines and inflammatory mediators. Regardless of the underlying mechanism they use, these various types of pathogen have all successfully evolved to evade and modulate the host defense systems. The mechanisms by which the different pathogens invade the host and cause infectious diarrhea are the topic of this Review.