Different ailments and diseases causes one to foam at the mouth, some are in born that is genetic, while some are caused by ones self.
Here are some of the ailments that causes foaming at the mouth:
Bell's palsy
Cranial nerve palsies
Epilepsy
Facial diplegia
Snake bite
Trauma
Tumours
Drug overdose

But thats not what we want to look at today, we want to look at how foaming at the mouth occurs. Now It depends on the scenerio.

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