Patients with severe infection can develop very low blood pressure. There are many mechanisms leading to this,and one of them appears to involve a hormone called vasopressin. In children as compared to adults, the mechanism and response to low blood pressure are different for reasons that are not clear. One possibility is the difference in the production and/or response to vasopressin. Vasopressin has been become part of the treatment of children with low blood pressure in the setting of severe infection, when other treatment has failed, but its use is on the basis of animal and adult studies. The exact timing and dose is uncertain. In this research study, the patients will receive standard treatment for sepsis and septic shock, and the investigators will measure the blood levels of vasopressin and a related compound called copeptin (both are required to understand the mechanism of control involved). Blood will need to taken from patients without any sepsis so as to be able to compare the values in health and in sickness. The patient groups the investigators have chosen for this are those children who will have blood taken anyway as part of their routine care. The aim of this study is to develop an understanding of the body's hormonal response (with respect to vasopressin) to severe infection in children. The long-term aim is to improve the care of critically ill children with severe infection by using the most appropriate dose of vasopressin at the most appropriate time.