1. Bayer Health Care, Pharma Research, Life Science Center Natural Products, Bayer Health Care, Animal Health, Research & Development, and Bayer Health Care, Pharma Research, Antiviral Research, P.O. Box 101709, D-42096 Wuppertal, Germany, and TÜV ISB, Engesserstrasse 4b, D-79108 Freiburg, Germany