1. Division of Infectious Diseases, Department of Internal Medicine, Centre Hospitalier Universitaire Vaudois, 1011 Lausanne,1 and
2. Department of Microbiology, Moyne Institut of Preventive Medicine, University of Dublin, Trinity College, Dublin 2, Ireland2
3. Division of Infectious Diseases, Hôpital Universitaire de Genève, 1211 Geneva,3 Switzerland, and