When you get sick with a cold, flu, or chest infection, your airways ramp up mucus production as a defense mechanism. Your body is essentially trying to trap and flush out the invading virus or bacteria. The cough that follows is the engine that moves all that extra mucus out of your lungs and throat. It feels miserable, but the whole process is your respiratory system doing exactly what it’s designed to do.
How Your Airways Make Mucus
Your lungs produce mucus all the time, not just when you’re sick. A thin layer of it lines the airways and works like a sticky conveyor belt, trapping dust, pollen, and germs before they can reach the deeper parts of your lungs. This mucus is about 95% water, with the rest made up of large proteins called mucins that give it its gel-like texture, plus small amounts of fats and other molecules.
Tiny hair-like structures called cilia sit just beneath the mucus layer. They beat in coordinated waves, similar to how your arms move during a breaststroke, pushing the mucus upward toward your throat. This system is sometimes called the “mucociliary escalator.” Under normal conditions, it works quietly in the background. You swallow most of this mucus without ever noticing it.
What Changes When You Get Infected
When a virus or bacterium infects your airways, the lining of your respiratory tract treats it as an injury. In response, your body converts more of its airway cells into mucus-producing cells, a process called goblet cell hyperplasia. Inflammatory signals trigger these cells to grow larger and multiply, dramatically increasing the volume of mucus being secreted. At the same time, immune cells like neutrophils rush to the site and release enzymes to fight the infection, adding cellular debris to the mix. This is why phlegm during illness looks and feels so different from the thin, clear mucus your body normally makes.
The infection itself can also damage the cilia that normally move mucus along smoothly. When those tiny structures are injured or slowed down, mucus builds up faster than it can be cleared passively. That buildup triggers the cough reflex, your body’s backup plan for getting the mucus out when the escalator isn’t keeping up.
What’s Actually in Your Phlegm
Phlegm during illness is a mix of mucus, dead immune cells, dead pathogens, and cellular debris. Neutrophils, the white blood cells that respond first to bacterial and viral threats, release enzymes designed to break down invaders. As those neutrophils die off, they become part of the phlegm you cough up. The thicker and more opaque your phlegm looks, the more of this cellular material it contains.
You may have heard that green or yellow phlegm means you have a bacterial infection and need antibiotics. This is largely a myth. The color comes from enzymes released by your own immune cells, not from the bacteria themselves. In one study of patients with respiratory flare-ups, 78% of samples that appeared clear or white still showed bacterial growth. Patient-reported phlegm color had a specificity of only 39% for detecting bacteria, meaning it was wrong more often than it was right. Color alone is not a reliable way to tell whether your infection is viral or bacterial.
Bronchitis vs. Pneumonia
Where the infection sits in your lungs affects how much phlegm you produce and how it feels. Bronchitis infects the bronchial tubes, the larger airways that branch off from your windpipe toward your lungs. These airways swell and fill with mucus, narrowing the passage. The hallmark symptom is a persistent, productive cough that may bring up yellow-green mucus as the infection progresses.
Pneumonia goes deeper, infecting the alveoli, the tiny air sacs responsible for transferring oxygen into your bloodstream. Instead of mucus buildup in the airways, fluid fills these sacs, making it harder to breathe. Pneumonia can also produce phlegm, but the more prominent symptoms tend to be shortness of breath, fever, and chest pain. Because pneumonia affects oxygen exchange directly, it’s generally more serious than bronchitis and more likely to need medical treatment.
Why the Cough Outlasts the Illness
One of the most frustrating parts of getting sick is the cough that lingers for weeks after you feel better. A post-viral cough typically lasts three to eight weeks after the acute infection clears. Here’s why: the infection may be gone, but the airway lining is still healing. Damaged cilia need time to regrow, and the airways remain inflamed and hypersensitive for a while.
Interestingly, this lingering cough is usually dry. By this stage, your body has stopped overproducing mucus because the infection is resolved. The cough persists because the nerve endings in your airways are still irritated and react to triggers like cold air, strong smells, or even deep breaths. It resolves on its own as the tissue fully heals.
Managing Phlegm While You’re Sick
Since coughing up phlegm is your body’s way of clearing infection, suppressing a productive cough isn’t always helpful. There are two main categories of over-the-counter cough medicines, and they do opposite things.
- Expectorants (like guaifenesin, the active ingredient in Mucinex) add water to the mucus in your airways, making it thinner and easier to cough up. They don’t stop the cough. They make it more effective.
- Cough suppressants reduce the cough reflex itself. These are better suited for a dry, nonproductive cough that’s keeping you up at night, not for the wet, phlegmy cough that’s actively clearing your lungs.
Staying well-hydrated works on the same principle as an expectorant: more fluid in your system helps keep mucus thin. Warm liquids, steam from a hot shower, and humidified air can all help loosen phlegm and make coughing less painful. If you’re producing a lot of phlegm, coughing it out rather than swallowing it can reduce nausea and keep your stomach more comfortable.
When Phlegm Signals Something Serious
Most productive coughs during a cold or respiratory infection are entirely normal and resolve within a few weeks. But certain changes in your phlegm deserve attention. Blood-streaked phlegm can happen from the sheer force of repeated coughing and is often harmless in small amounts. Coughing up larger quantities of blood, roughly half a cup or more in 24 hours, is considered a medical emergency. Even smaller amounts can be dangerous if you already have underlying lung or heart disease.
Phlegm that persists beyond eight weeks, phlegm accompanied by high fever and difficulty breathing, or a sudden worsening after you’d started to improve can all point to complications like pneumonia or a secondary bacterial infection layered on top of an initial viral illness.