For most people the first sign of being sick is a runny nose. It's on your face where everyone can see it, and you can't breathe regularly without sniffling. But having a runny nose does not automatically make you sick by default. There are hundreds of things that can cause your nose to run, but generally they fall into a few categories.
Infections
Yellow or green mucus is the trademark of an internal infection. This is the immune system's defense against germs that cause colds and actually appears two to three days after the germs cause the infection. This kind of infection definitely means that you actually are sick and might want to visit your doctor if conditions persist for longer than seven days.
Allergens
Mucus builds up in the nose in response to the introduction of allergens. Its another one of your immune system's tools to defend and protect your body from infection.
Objects
Small things can get easily lodged into the nostril cavity. Dust, pollen, fur and hair are all small enough to float through the air and be inhaled. If the objects are not fully inhaled and become stuck in the nostril cavity, the body's immune system goes into defend mode to eliminate the infection. Upon sneezing, the problem will be eliminated and the nose will return to its normal condition.
