spit, contempt and illnesses

Why do we spit to express our contempt? Where does it come from?
moreover, i know that spitting on the ground was forbidden during the SARS epidemic . Why is spitting dangerous? I guess i won’t breath or touch it. any clue?

I would guess it would have something to do with people walking barefoot on the ground. If you had an open wound on your foot and the germy spit got in, you could get sick. Also little kids wouldn’t know not to touch or eat it, and they could get sick.

I don’t know about the first part of the question.

Ever had someone spit in your face? It’s not that pleasant.

I am not a social anthropologist but:

Spitting originally is the act of rejecting something distasteful. It’s merely a step toward metaphor to spitting as an expression for something you find intellectually, morally or idiosyncratically (no, that’s not a word until now) unacceptable. I’d like for a word expert to weigh in on when “unpalatable” expanded its meaning, but I’d bet it was after spitting became known as an expression of opinion.

How would you like to walk barefoot in a Major League baseball players’ dugout?

Those guys spit incessantly. I think it’s a macho thing. And what is more manly than hocking a mouthful of tobacco juice, especially when the camera’s turned on you.

Tony Canigliaro (spelling?) used to chew Tootsie Rolls so he could “spit brown.”