Answers: If you swallow large quantity of it, it may upset your stomach, but it's not truly harmful. It will run into your stomach, not back into your respiratory system, so you will not cough it up again. The germs it may contain will usually be specific to the respiratory system, so they will not affect your digestion and will be kill by your stomach acid. All surrounded by all, though, it's still a better perception to spit it out when you can just to avoid upset stomach.
You really shouldn't. Try to spit it out surrounded by the sink or toilet. If you swallow it, you will just cough it up again or choke on it.