Who am I?

I am yellow, sometimes green, sometimes bloody, sometimes thick, sometimes runny. I am found on every continent. I prefer to stay in, but when something is wrong I often come out in large quantities. Everyone has me in them, but no one wants me on them. I have an important function, but that does not make me any less slimy or gross. Products have been developed specifically to remove me when I come out. I am not popular when I come out in crowded places. In fact, sometimes I am left on the ground when my usefullness is expelled. Sometimes swallowed, often spit out. Who am I?

If you can answer the above question you have a clue as to why I am not writing anything else. Basically, I have a lot of the answer and am currently trying to get rid of it. Thank goodness for chicken soup!