Gastrointestinal illness following natural disasters is a common occurrence and often results from the disruption of potable water supplies. The risk for outbreaks of gastrointestinal illness is higher in developing countries because of fewer available resources and poorer infrastructure. But industrialized countries are not immune from this problem, as demonstrated by an outbreak of gastroenteritis from norovirus that followed in the wake of Hurricane Katrina in 2005. Rates of gastrointestinal illness following natural disasters are influenced by the endemicity of specific pathogens in the affected region before the disaster, the type of disaster itself, the availability of health care resources, and the response by public health personnel after the disaster. Ensuring the uninterrupted supply of safe drinking water following a natural disaster, like adding chlorine, is the most important strategy to prevent outbreaks of gastrointestinal illness.