Leprosy, also called Hansen's disease, is a disease produced by infection with a bacterium called Mycobacterium leprae. It has a long incubation period (time between getting infected and developing the disease), usually several years. Leprosy is a disease of low incidence in the United States; only 108 new cases were reported in 1999. If you are called to give advice on the type of study design to be conducted to assess the possible association between several exposures and leprosy, which design do you think would be the best choice?