Snot.

quote:

Diseases involving mucus
Generally mucus is clear and thin, serving to filter air during inhalation. During times of infection, mucus can change color to yellow or green either as a result of trapped bacteria, or due to the body's reaction to viral infection.

In the case of bacterial infection, the bacterium becomes trapped in already clogged sinuses, breeding in the moist, nutrient-rich environment. In this case, the clogged sinuses are a result of some other condition (such as allergies) and the bacterial infection is secondary to this original cause. When two different colors of common bacteria become mixed (such as yellow Staphylococcus aureus and blue Pseudomonas pyocyanea) the resulting shade is often green. Antibiotics may be used fruitfully to treat the secondary infection in these cases, but will generally not help with the original cause.

In the case of a viral infection such as cold or flu, the first stage of infection causes the production of a clear, thin mucus in the nose or back of the throat. As the body begins to react to the virus (generally one to three days), mucus thickens and may turn yellow or green. In these cases, antibiotics will not be useful, and are a major source of misuse. Treatment is generally symptom-based; the only cure is to allow the immune system to fight off the virus over time.