It is hypothesized that prior illness exacerbates heat stroke (HS) in otherwise healthy organisms by augmenting hyperthermia during heat exposure or deactivating cellular pathways that protect against organ injury. To test these hypotheses, we injected telemetered male C57BL/6J mice with lipopolysaccharide (LPS; 50 µg kg −1 I.P.) or polyinosinic:polycytidylic acid (PIC; 100 µg I.P.) as a bacterial or a viral mimic, respectively, with saline (SAL; equivalent volume) as a control. Mice recovered for 48 or 72 h before HS (maximal core temperature = 42.4 • C). Platelet counts, cytokines, chemokines and organ injury were determined 48 or 72 h after injection (without heating) or at maximal core temperature and at 1 day of recovery from HS. In the absence of heat, PIC induced more robust signs of sickness and increased cytokines and chemokines (TNF-, RANTES, IP-10 and MIP-1) at 48 h, which was not observed with LPS (P < 0.05). Responses of both groups recovered by 72 h, although low platelet counts persisted after PIC (P < 0.05). Heat-induced hyperthermia was similar among mice injected with SAL, LPS and PIC; however, PIC-injected mice displayed more severe responses during recovery from HS, with reduced survival (48 h, 70 versus 100%; P < 0.05), deeper and longer post-HS hypothermia, greater reductions in platelets, elevated RANTES, IP-10, IL-6 and TNF-and greater duodenal injury (P < 0.05). By 72 h, survival from HS was no longer reduced in PIC-injected mice, although hypothermia, the reduction in platelets and elevated cytokines persisted. Our data indicate that prior illness exacerbates the severity of HS in the absence of signs of illness at the time of heat exposure and suggest that this is attributable to persistent coagulation and inflammatory reactions that might be targets for intervention to improve recovery.