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Fever
Medical Condition
Use 'fever' to describe an elevated body temperature caused by illness, not just feeling warm. 
Common Expressions
'Running a fever' means currently having a high temperature, while 'breaking a fever' means the fever is subsiding. 
Severity Indicator
Adjectives like 'high', 'low', 'spiking' describe fever's intensity and can signal the seriousness of illness. 