Cover photograph (Copyright © 2004, American Society for Microbiology. All Rights Reserved.): Fluorescent images of cells infected by a vaccinia virus conditional lethal mutant showing extracellular virions (green) at the tips of actin tails (red). DNA of nuclei and adjacent virus replication factories are stained blue. (See related articles in March 2004: vol. 78, issue 5, pages 2348 and 2357.).