1. Department of Epidemiology, College of Public Health, University of Iowa, Iowa City, IA 52242, USA
2. Department of Obstetrics/Gynecology, College of Medicine, University of Iowa, Iowa City, IA 52242, USA
3. Veterans Affairs Medical Center, Iowa City, IA 52240, USA
4. Department of Experimental Virology, Institute of Hematology and Blood Transfusion, U nemocnice 1, 128 20 Prague 2, Czech Republic