Rapidly giving intravenous fluid to prevent or treat shock (fluid resuscitation) is one of the commonest treatments given to critically ill patients. Current guidelines recommend crystalloid solutions but it is unknown whether any particular crystalloid is better than others. This trial will determine whether the use of one of two crystalloid fluids, saline or PlasmaLyte, reduces the risk of organ injuries, such as kidney failure, and improves patients chances of surviving critically illness.