Cover photograph (Copyright © 2006, American Society for Microbiology. All Rights Reserved.): The protozoan pathogen >Toxoplasma gondii actively invades host cells using a unique mechanism involving polarized secretion of proteins and an intrapellicular motor system. One means of observing distinct stages of invasion is to use a differential staining method called the red-green assay. The visually enhanced fluorescence/phase overlay image shows parasites that are extracellular (red), intracellular (green), or are invading (green and red) human fibroblast cells (blue; nucleus). (>See related article on page 2174.)