Proposed plasticity in BMSC response and function during infectious challenge. To decrease infectious burden (left white triangle), the host requires intact immunity (left black triangle). That is, level of pathogen burden in the host inversely correlates with host immune competency. BMSCs may act as pro-inflammatory agents in the initial stages of the host response to infectious challenge, thereby decreasing pathogen burden by augmenting antimicrobial immune responses. However, high-level, persistent systemic inflammation (right white triangle) ultimately results in decreased host survival (right black triangle). Therefore, BMSCs may assume an immunomodulatory function to dampen the pro-inflammatory response associated with host immune response to pathogen. In this regard, BMSCs might maintain immune homeostasis after infectious challenge, by preserving host integrity through mediating initial pro-inflammatory, antimicrobial effects and then shifting function to attenuate inflammation and to augment tissue repair.