1. Department of Clinical Sciences “Luigi Sacco,” Section of Infectious Diseases, University of Milan, Milan, Italy.
2. Section of Microbiology, Department of Molecular Biology, University of Siena, Siena, Italy.
3. Division of Epidemiology and Prevention, Institute of Human Virology, Baltimore, Maryland.
4. Department of Experimental Medicine and Biochemical Sciences, Section of Infectious Diseases, University of Perugia, Perugia, Italy.