1. Section of Pulmonary, Critical Care, Sleep and Allergy, Department of Medicine M/C 719, University of Illinois at Chicago, 840 S. Wood Street, Chicago, IL 60612-7323, USA
2. Institute of Human Virology, University of Maryland School of Medicine, Baltimore, MD 21201, USA
3. Department of Health, University of Louisville, Louisville, KY 40202, USA
4. Department of Biological Science, University of Virginia Tech, Blacksburg, VA 24061, USA