Having him use listerene would be a good idea as far as protecting yourself from getting a bacterial infection if he is unhygenic with dental care, but if he does have oral herpes (cold sores/fever blisters), it wouldn't give any protection from that. Also, keep in mind that it is estimated that 80% of the population has oral herpes (but most people don't get symptoms), and it is possible to spread it even if there are no symptoms present.
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