Cover photograph (Copyright © 2006, American Society for Microbiology. All Rights Reserved.): Colorized electron microscopic image of vaccinia virus entering a cell through the plasma membrane. Fusion of the viral membrane with the cell membrane is visualized by surface immunogold (yellow) labeling of the mound above the core (purple), which has entered the cytoplasm. Vaccinia virus also enters cells by an efficient low-pH endocytic pathway. (See related article on page 8899.))