Cover photograph (Copyright © 2005, American Society for Microbiology. All Rights Reserved.): An electron cryomicrograph and interpretation show frozen hydrated pleomorphic Tacaribe arenavirus particles with diameters of ~400 to ~2,000 Å. Surface GP-C spikes, composed of distinctive head and stalk regions (violet), extend through the phospholipid bilayer (orange). A concentric layer of density adherent to the inner bilayer leaflet was assigned to protein Z (red), and a second concentric layer at a shorter radius was assigned to paracrystalline nucleoprotein molecules (blue). (See related article on page 3822.)