Severe infection can cause circulatory failure and reduced tissue perfusion; this is known as 'septic shock'. This complication of infection is a major cause of mortality worldwide, with a case fatality rate in developed countries of 25 - 30%. Treatment with steroids (cortisone-like medicines) can reduce the inflammation induced by infection and improve the circulation, but it remains uncertain if their use improves survival overall, as they carry a risk of serious complications in the critically ill. This is a placebo-controlled study of low-dose hydrocortisone in critically ill patients, to determine if this treatment improves survival and quality of life up to six months after the initial illness. If it is effective, treatment of severe infection for thousands of patient worldwide will be improved.