When the human body is invaded by an infectious organism (such as a bacterium, fungus, or virus), it fights back with a number of defense systems, including activation of numerous immune cells and molecules that help trap and kill the organism. The battle between the infection and the body's defenses can escalate, and the defenses themselves can unfortunately damage or impair vital organ function. Sepsis is a broad term that describes the syndrome of the body trying to fight a potentially serious infection. Sepsis is termed severe sepsis when this battle leads to acute vital organ damage. For example, a patient trying to fight pneumonia develops severe sepsis if the pneumonia is complicated by damage to the brain, heart, liver, lungs, or kidneys. Ultimately, death occurs in 20% to 50% of those who have severe sepsis. The most profound form of severe sepsis is septic shock (sepsis associated with hypotension [low blood pressure] despite replacement of intravenous fluids). The June 17, 2009, issue of JAMA includes an article about septic shock.