Cover photograph (Copyright © 2001, American Society for Microbiology. All rights reserved.): Colorized scanning electron micrograph of an RK₁₃ cell at 16 h after infection with vaccinia virus. Protruding from the cell body are thick specialized microvilli, each tipped with a virus particle. The microvilli, formed by actin polymerization at sites where intracellular enveloped virions fuse with the plasma membrane, greatly enhance cell-to-cell virus spread. The cell and virions are pseudocolored red and green, respectively. The micrograph was prepared by A. Weisberg. (See related article on page 11651.)