Most of that seemingly endless mucus during a cold is made by your own nose and sinus lining ramping up their normal production as part of the immune response to the virus. You actually make around 1–2 quarts of mucus every day even when healthy; a cold just makes that production thicker, more visible, and harder to clear.

What mucus is made of

  • Mucus is mostly water plus special gel-forming proteins called mucins , along with salts, fats, and immune proteins.
  • Specialized “goblet” cells and submucosal glands in your nasal passages, sinuses, and airways constantly secrete this mixture to keep surfaces moist and protected.

Where “all that” mucus comes from

  • When a cold virus infects your nose and sinuses, the immune system releases chemicals (like histamine) that inflame the lining and tell those glands and goblet cells to pump out more mucus.
  • Because mucins are made in small amounts and then swell with water, most of the bulk of your snot is actually body water grabbed into that gel, not a whole new mysterious substance.

Why it feels like a never-ending supply

  • Even without a cold, your respiratory tract produces over a liter of mucus a day, most of which you quietly swallow without noticing.
  • During a cold, production increases and the drainage system gets clogged by swelling, so instead of moving silently to your throat, mucus backs up, drips, and needs to be blown out or coughed up.

Why the color and thickness change

  • Early in a cold, mucus tends to be clearer and more watery because it is mostly water and mucin with relatively few immune cells.
  • As your immune system sends white blood cells into the area and they trap and digest viral particles, the mucus can thicken and turn yellow or green—this usually reflects immune activity, not automatically a bacterial infection.

Quick note on throat mucus and “postnasal drip”

  • A lot of what you feel “in your chest” or back of your throat is actually nasal and sinus mucus draining downward (postnasal drip), not always mucus made deep in the lungs.
  • Cilia—tiny hairlike structures—wave in coordinated strokes to move mucus toward the throat, where it is swallowed and processed in the stomach, helping remove viruses and debris from the airways.

TL;DR: The “endless” mucus during a cold is your usual daily mucus production turned up to high and made more obvious—produced by goblet cells and glands in your nose, sinuses, and airways, filled mostly with water plus mucin and immune cells, then poorly drained because everything is inflamed.

Information gathered from public forums or data available on the internet and portrayed here.