The feverish child
Fever is a non-specific immune response common to trivial viral infection and life-threatening sepsis alike, so the NICE traffic-light system exists to convert a set of clinical observations into a structured risk stratification that predicts which febrile child is septic.
First principles
Fever itself tells you almost nothing about severity
Fever is produced by pyrogens (cytokines such as IL-1 and IL-6) resetting the hypothalamic set point, a response mounted equally by a self-limiting viral URTI and by invasive bacterial sepsis. Because the height of the temperature does not reliably distinguish trivial from life-threatening illness, especially in young children, assessment must shift from 'how high is the fever' to 'what is the physiological response to the fever': heart rate, respiratory rate, perfusion, conscious level and behaviour. This is why a structured system of clinical features, not the thermometer reading, drives triage.
Educational content pending clinical review. Not medical advice.