proximately 0.1-2 μM) that is significantly lower than that observed in the extracellular medium (mM) due to Ca 2+ transporters and channels [8] [9] [10] [11] . Similar to the eukaryotic systems, P-type ATPase Ca 2+ efflux pumps have been characterized from Synechococcus and Flavobacterium. A Ca 2+ transporter of S. pneumoniae is involved in Ca 2+ -DNA uptake, lysis, and competence [12, 13] . Uptake of Ca 2+ and other divalent cations can also accompany uptake of phosphate by the phosphate transport system of E. coli. Furthermore, it has been reported that bacteria contain Ca 2+ binding proteins that are essential for cell adhesion and communication [14] [15] [16] [17] . Viruses, on the other hand, utilize the universal Ca 2+ signal to create a specific cellular environment to achieve their own purposes (Figure 1(b) ). Ca 2+ plays important roles in viral gene expression, post-translational processing of viral proteins, virion structure formation, virus entry, and virion maturation and release. As shown in Figure 1 (b), the interplay between viruses and Ca 2+ in the infected cell falls generally into three major categories: (1) viral proteins directly or indirectly disturb Ca 2+ homeostasis by altering membrane permeability and/or manipulating key components of the Ca 2+ -signaling apparatus; (2) viral proteins directly bind to Ca 2+ for structural integrity or functionality; and (3) critical virus-host interactions depend on cellular Ca 2+ -regulated proteins or pathways.