As such he bathes and eats separately and never laughs or shows excessive emotion to avoid accidentally getting spit on his friends.
This way the child gradually gets used to poisons and becomes the perfect assassination tool - the "Poison Princess" whose every kiss, tear and even touch bring death. Even mixed in the milk that they feed the newborn.
And poor Kagerou ends up subjected to a Break the Cutie process that's bad even by this VERY dark series's standards. In this version, she is a tragic figure who is desperately in love with a man she can never have because consummating that love would kill him and he's the future clan chief, and she's forced to watch him fall in love and with another woman from an enemy clan.
If they're really lucky they'll have enough control of their power/curse to turn it on and off, or at least be able to live in urban areas by wearing full body outfits or hazmat suits. If they aren't evil, they'll usually get far, far away from anyone who they might hurt, like a modern day Medusa or King Midas.