The lining of your throat and nasal passage produces mucus. This is your immune system's first line of defense. It traps intruders like bacteria and viruses in a thick, wet substance.

When you produce normal amounts of mucus, you do not notice it dripping down the back of your throat and into your stomach. When you produce more mucus, if the mucus is thicker than normal, or if your throat is more sensitive, you may feel the mucus as it trickles down the back of your throat.

And no, it is not always contagious.

Increased post nasal drip can be caused by many things including allergies, pregnancy, certain medications like some birth control and blood pressure medications, cold weather, dry air, foods and fumes from smoke or cleaning products, inhaled irritants, inflammation in the nose and sinuses, and acid reflux.

But, post nasal drip can also be a symptom of infections, such as influenza, the common cold, or other sinus infections. In these cases, it is not the post nasal drip itself that is contagious. Rather, it is the infection causing the post nasal drip that can be contagious.

• Northwestern Medicine Otolaryngologist Stephanie Shintani Smith, MD, MS