Physicians use gastrointestinal symptoms to judge the severity of a person's radiation exposure. A high dose of radiation can destroy the intestinal barrier, a thin layer of cells that acts as the interface between our bodies and the millions of bacteria that live in our intestines. Normally these bacteria work with us, breaking down our food and regulating our mood, but with the intestinal barrier gone, the body has trouble absorbing water or nutrients in food. Plus, the bacteria can stray and infect other parts of the body. As a result, a person with radiation poisoning often shows severe dehydration, nausea, vomiting, and intestinal pain. Without treatment, a patient will die—usually of infection—within two weeks.