Affected by rabies (a deadly disease), or extremely fanatical and intense about something.
From Latin 'rabidus' (raging), related to 'rabies' (madness). The disease name and adjective share the same root, both describing uncontrollable, wild behavior seen in infected animals.
Rabies is one of the few diseases with a nearly 100% fatality rate once symptoms appear, which is why calling someone a 'rabid fan' carries dark echoes of something genuinely dangerous and out of control.
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