Naturally, Fennel is immune to viruses, toxins, etc. His crest and chest fur (the V and X shaped things) expel an intense heat that burn harmful bacteria instantly. When he's wounded with serious cuts though, he can get poisoned or ill. He'll have a depressed expression and the chest/crest fur glow of the color of the poison in him. They expel whatever toxin or virus that has entered his body.

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