Why does mucus thicken when you have a cold?

During flu season, patients often ask: "Does thickened mucus mean my cold is getting better? I have been taking medication, but why is my mucus thickening and why does it turn into sinusitis?" A common cold usually starts with viruses entering the nasal membrane. At such instance, the body immune system responds by attacking the intruding viruses. If you have a well functioning immune system, the viruses can be eliminated and you will be free from a cold. If unfortunately, you have been pulling all-nighters, or have stress from work, or simply not in a good condition, then your immune system may not be able to eliminate the viruses completely. As a result, the viruses grow exponentially and symptoms such as stuffed nose, running nose, sore throat, cough, and ...etc. may appear. It would then usually take 7-10 days before your upper respiratory and nasal mucosa membrane return to normal.
Using medications to reduce nasal discharge will at the same time slow down the nasal mucociliary movement function. This provides an optimal environment for the viruses.
In the first stage of virus invasion, the nasal membrane will attempt to flush out the invaders through sneezing and secretion of nasal discharge. This is the result of our body defense system. However it may interrupt with our daily life and work, most people dislike such mechanism and the increased nasal discharge. To minimize the effect of nasal discharge, most people turn to medications. With the use of medications, secretion of nasal discharge is reduce, but at the same time, the medications slow down the nasal mucociliary movement function. This provides an optimal growth environment for the viruses. This is the reason why many people complain about thickening mucus and sinusitis after taking medication. If you blow your nose forcefully during a cold and further damages the nasal mucosa membrane, or blowing mucus into your sinuses, or the viruses cause sinus mucosa to swell and impaired the mucociliary movement function, you may likely induce bacterial infection. Bacterial infection would thicken the mucus and turn mucus into a yellow-greenish color. This is clinically referred as acute sinusitis. If it lasts more than 7 days, 1-2 weeks of continued use of antibiotics is recommended.
Diligent use of warm saline nasal irrigation can effectively remove cold viruses.
When you experience running nose during a cold, the best solution is to keep warm, drink more water, sleep early, and improve your body immune system. At the same time, active use of warm nasal irrigation can effectively remove cold viruses. If you must take medications, do so with precautions to avoid thickened mucus and acute sinusitis.