Having the symptoms of fever.
“He was wracked with febrile sweats most of the week.”
But fever, central to the definition of febrile, can also mean agitated and excited (Saturday Night Fever ain’t about the flu). This secondary definition of fever has informed our usage of febrile, making it acceptable to say:
“His frustration became a febrile anger.”
“The febrile crowd became a mob when they heard Ozzy wouldn’t be appearing.”