The common cold is caused by a viral infection, most often by rhinoviruses. More than 200 different viruses can trigger cold symptoms, including parainfluenza viruses and seasonal coronaviruses, but rhinoviruses are responsible for the majority of cases. The virus itself doesn’t make you feel terrible, though. Most of what you experience as a “cold” is actually your own immune system fighting back.
The Viruses Behind a Cold
Rhinoviruses are the leading cause of the common cold. The name literally means “nose virus,” and they thrive in the slightly cooler temperatures of your nasal passages, around 33 to 35°C, compared to your core body temperature of 37°C. That preference for cooler environments is one reason colds tend to settle in your nose and throat rather than deeper in your lungs.
Other viruses that cause colds include parainfluenza viruses, respiratory syncytial virus (RSV), adenoviruses, and seasonal coronaviruses (not to be confused with the virus that causes COVID-19). Because so many different viruses can cause a cold, you can catch several per year, and your body never builds lasting immunity against all of them.
How the Virus Gets In
Cold viruses spread in three main ways. The most common route is through tiny droplets released when an infected person coughs or sneezes. You breathe these in, and the virus lands on the lining of your nasal passages. Viruses can also survive on surfaces like doorknobs, phones, and countertops. Touch a contaminated surface, then touch your eyes, nose, or mouth, and you’ve given the virus a direct path in. Close personal contact, like shaking hands, works the same way.
Once a rhinovirus reaches your nasal lining, it latches onto a specific docking site on the surface of your cells. The virus locks into this receptor, which triggers structural changes in the virus particle, allowing it to inject its genetic material into the cell. From there, the virus hijacks your cell’s machinery to make copies of itself. Those copies burst out and infect neighboring cells, and the cycle repeats.
Why You Feel Sick
Here’s the part most people don’t realize: the virus itself causes relatively little direct damage to your airway cells. The runny nose, sore throat, congestion, and fatigue you feel are mostly caused by your immune system’s reaction to the infection.
When your body detects the virus, immune cells release signaling molecules called cytokines. These cytokines trigger inflammation in your nasal passages, which causes swelling and increased mucus production. That’s your stuffy, runny nose. Cytokines also act on nerve endings and the brain’s temperature control center, which can make you feel chilly and achy. They’re responsible for headaches too. Administering these same immune molecules to healthy people in experiments produces headache, fatigue, and general malaise even without any virus present.
Cytokines also trigger what researchers call “sickness behavior,” a cluster of symptoms including fatigue, difficulty concentrating, low mood, and loss of interest in food or activities. That heavy, foggy feeling when you have a cold isn’t the virus wearing you down. It’s your immune system redirecting your energy toward fighting the infection.
Why Cold Weather Matters
The old idea that “cold weather gives you a cold” isn’t entirely wrong, just incomplete. You need a virus to catch a cold, but temperature plays a real role in whether that virus takes hold.
Research from Yale University found that the cooler temperatures inside your nasal cavity (around 33°C) significantly weaken your local immune defenses. At core body temperature (37°C), airway cells mount a much stronger antiviral response, producing higher levels of protective proteins that limit viral replication. At the cooler nasal temperature, the activity of key immune sensors drops by 20 to 65 percent, giving the virus a window to multiply before your defenses catch up.
Winter also pushes people indoors into closer contact, and heated indoor air tends to be dry, which can compromise the protective mucus layer in your nose. All of these factors converge to make cold season a real phenomenon, not just a coincidence.
Risk Factors That Increase Susceptibility
Not everyone exposed to a cold virus gets sick. Your baseline health, stress levels, and sleep habits all influence whether an exposure turns into a full-blown cold.
Sleep is one of the strongest predictors. In a study of 164 healthy adults, researchers measured participants’ sleep with wrist monitors for a week, then deliberately exposed them to rhinovirus through nasal drops. People who slept six hours or less per night were more than four times as likely to develop a symptomatic cold compared to those who slept seven hours or more. That’s a striking difference from just one hour less sleep.
Young children catch the most colds, typically six to eight per year, because their immune systems haven’t encountered many viruses yet. Adults average two to three colds annually. Chronic stress, smoking, and close contact with young children or large groups of people all raise your odds.
How Long You’re Contagious
The incubation period for a cold is short: between 12 hours and three days after exposure. You can start spreading the virus a day or two before you even notice symptoms, which is one reason colds move so efficiently through households and offices.
You’re most contagious during the first three days of feeling sick, when symptoms are at their worst and viral shedding is highest. But you can remain contagious for up to two weeks. That lingering cough at the tail end of a cold may still carry enough virus to infect someone nearby.
Cold vs. Flu: Key Differences
Because colds and flu share symptoms like sore throat, cough, and congestion, it’s easy to confuse them. The differences come down to intensity and speed of onset. Flu hits hard and fast, often with sudden fever, significant body aches, and exhaustion. A cold builds more gradually and stays milder. Runny or stuffy nose is more characteristic of a cold than the flu.
The stakes are also different. Colds rarely lead to serious complications. Flu can progress to pneumonia, bacterial infections, and hospitalization, particularly in older adults, young children, and people with chronic health conditions. If your symptoms come on suddenly with high fever and severe body aches, that pattern points more toward flu than a cold.