Spitting on public transit
The driver ought to be disciplined, but hopefully not too harshly. As disgusting as it is, spitting back was probably an instinctive reaction. Unfortunately, this kind of incident happens often enough to bus drivers that they really have to have a policy about it.
Note to spitters everywhere: on the classy scale, it ranks somewhere just below picking your nose and just above scratching your groin and then offering to shake someone’s hand. Not good behavior for making friends and influencing people.