If someone with an infectious disease knowingly infects another, whether it be by unprotected sex without disclosure or by spitting, then that is malicious intent to harm and is already well covered by law.
And what if they didn't know they had a disease? It doesn't matter - forcing someone to share your bodily fluids in such an aggressive manner should be a crime. And of course the punishment should fit the crime, but at a minimum, spitting is equal to minor physical assault.
