The Mandan, as a nation of people, were hit by numerous waves of smallpox and cholera, whooping cough, measles, and pivotally, epidemics of Norwegian rats that came in on riverboats.  At first, the Mandan and Hidatsa, who had never seen a brown rat, were entranced and even happy to have the rats, because they ate the deer mice that had long plagued their earthen lodges.