Pliny the Elder (Caius Plinius Secundus) once said...
All men carry about them that which is poyson to serpents: for
if it be true that is reported, they will no better abide the
touching with man's spittle than scalding water cast upon them:
but if it happed to light within their chawes or mouth,
especially if it come from a man that is fasting, it is present