1. Enteric Diseases Epidemiology Branch, Centers for Disease Control and Prevention; Atlanta GA 30333
2. Departments of Epidemiology and Bacteriology; Statens Serum Institut; Copenhagen DK-2300 Denmark
3. Department of Epidemiology and Surveillance; Rijksinstituut voor Volksgezondheid en Milieu; 3721 MA Bilthoven The Netherlands
4. Division of Foodborne, Bacterial and Mycotic Diseases; Centers for Disease Control and Prevention; Atlanta GA 30333