You catch a cold when a virus enters your body through your nose, mouth, or eyes, almost always after contact with an infected person. The primary route is breathing in tiny virus-laden droplets that someone else has coughed, sneezed, or even exhaled into the air around you. The second most common route is touching a contaminated surface and then touching your face. From the moment the virus lands on your nasal lining, symptoms typically appear within 24 to 72 hours.
Airborne Droplets Are the Main Route
When someone with a cold sneezes, coughs, or talks, they release microscopic droplets carrying the virus. Breathing these in is the primary way colds spread. The droplets can travel several feet, which is why close contact with a sick person (sitting next to them at work, sharing a car, sleeping in the same room) creates the highest risk. Smaller particles can linger in poorly ventilated indoor spaces, which partly explains why colds spread more easily when people gather indoors during cooler months.
The Hand-to-Face Route
The second major pathway is what researchers call the hand-nose-hand route. Someone with a cold blows their nose or covers a cough, leaving virus on their hands. They touch a doorknob, a phone, a shared keyboard. You touch the same surface, then rub your eye or nose, delivering the virus directly to the mucous membranes where it can take hold.
Cold viruses can survive on indoor surfaces for up to seven days, though they’re typically only infectious for about 24 hours. They last longer on hard, nonporous materials like plastic, stainless steel, and countertops. On soft, porous surfaces like tissues or fabric, they die off faster. This means the grocery cart handle or the office coffee pot is a more likely source of transmission than someone’s sweater.
What Happens Once the Virus Gets In
More than 150 types of rhinovirus cause the majority of colds, and they’re remarkably efficient. Studies suggest that an extremely small amount of virus, less than a single infectious unit in lab terms, can infect half the people exposed to it. You don’t need a big dose to get sick.
Once the virus reaches your nasal lining, it latches onto specific proteins on the surface of your cells, gets pulled inside, and begins replicating. Your immune system detects the invasion and mounts an inflammatory response. That inflammation is what causes most of your symptoms: the stuffy nose, sore throat, and sneezing aren’t the virus destroying tissue so much as your body fighting back.
Symptoms usually begin with a scratchy or sore throat, followed by sneezing, a runny nose, congestion, and general fatigue. You’re actually contagious before you feel anything, starting a day or two before symptoms appear. The most contagious window is the first three days of feeling sick, when viral shedding peaks. Total contagiousness can last up to two weeks.
Why Cold Weather Actually Matters
The old idea that being cold gives you a cold has more truth to it than scientists used to think. It’s not that cold air contains viruses. It’s that cold air weakens your nose’s first line of defense.
Research from Harvard Medical School found that when people moved from room temperature (about 74°F) into near-freezing air (about 40°F) for just 15 minutes, the temperature inside their noses dropped by roughly 9°F. That small change had a significant effect: nasal cells released nearly 42% fewer of the tiny immune particles they normally deploy against incoming viruses. The antiviral proteins in those particles were also impaired. In other words, a cold nose is a less defended nose.
This helps explain why cold and flu season reliably peaks in winter. It’s not just that people spend more time indoors together. There’s a genuine biological reason your respiratory defenses weaken in cold air.
Dry Air Makes It Worse
Low humidity is the other seasonal factor that boosts transmission. Animal studies found that viral spread was most efficient at the two lowest humidity levels tested (20% and 35%) and dropped substantially at 50%. At 80% humidity, no transmission occurred at all. Cold temperatures showed a similar effect: animals housed at 41°F shed virus for 40 hours longer than those at 68°F, and at 86°F the virus didn’t spread between animals at all.
Two things happen in dry air. First, water evaporates quickly from respiratory droplets, shrinking them into lighter particles that stay airborne longer and travel farther. Second, dry air can dehydrate the mucous lining in your nose, slowing down the tiny hair-like structures that normally sweep viruses out before they can infect cells. Indoor heating in winter dries air even further, creating ideal conditions for cold viruses. Keeping indoor humidity between 50% and 80%, and temperatures above 68°F, may slow transmission.
How to Reduce Your Risk
Handwashing with soap and water remains one of the most effective defenses. Alcohol-based hand sanitizers also work well. Head-to-head trials have produced mixed results comparing the two: some found sanitizer slightly more effective (reducing respiratory infections by about 13% compared to soap alone in one study of childcare centers), while others found no meaningful difference. The consistent finding is that either method significantly reduces transmission compared to doing nothing. The key is doing it frequently, especially after touching shared surfaces and before touching your face.
Beyond hand hygiene, a few practical steps lower your odds. Keep distance from people who are actively sneezing and coughing when possible. Ventilate indoor spaces, since stagnant air lets viral particles accumulate. In winter, consider using a humidifier to keep indoor air from dropping below 40% to 50% relative humidity. And if you’re the one who’s sick, the first three days of symptoms are when you’re spreading the most virus, so that’s when staying home has the biggest impact on the people around you.