Inclusion Criteria:  Patients above the age of 18 years old. Patients with sepsis diagnosed within 48 hours of enrollment. For the purpose of this protocol, sepsis onset is defined as administration of intravenous antibiotics in a patient that the treating critical care physician feels is ill from sepsis. All qualifying patients must have a sequential organ failure assessment (SOFA) score of 2 or greater at the time of enrollment. Patients requiring mechanical ventilation with an expected duration of mechanical ventilation of >= 2 days. Patient maximum baseline documented temperature (within previous 24 hours) < 38.3°C.  Exclusion Criteria:  Patients without legally authorized representative able to provide informed consent. Patients with contraindication to core warming using an esophageal core warming device, surface warming using conductive heat transfer, and forced air warmer. Patients known to be pregnant. Patients with <40 kg of body mass. Patients with comfort care only status. Patients with contraindications to hyperthermia treatment (acute stroke, acute traumatic brain injury, acute spinal cord injury, post-cardiac arrest, multiple sclerosis, or sickle cell disease). Expected continuous dialysis within 48 hours after enrollment.