Commensal symbionts may become pathogens upon escaping their habitat. In the gut, Bacteroides fragilis protects against colitis through induction of interleukin 10 (IL-10) by CD4(+) T cells. When intestinal integrity is disrupted, B. fragilis and colonic contents escape into the peritoneum, causing abscesses and bacteremia. Whether the virulence mechanisms employed by B. fragilis during infections differ from those employed for symbiosis during commensalism is unknown. We demonstrate T cell-independent IL-10 production in response to B. fragilis during its pathogenic interactions with the host, and demonstrate the ability of the whole organism to activate Toll-like receptor 2-mediated MyD88 signaling in macrophages. Upon challenge with B. fragilis, mortality rates and serum proinflammatory cytokine levels were higher among IL-10(-/-) mice than among wild-type mice. Deaths were due to exuberant proinflammatory responses, not increased bacterial burden. During infection or commensalism, induction of IL-10 by B. fragilis is critical to this microbe's interactions with the immune system.