COVID-19 spreads primarily through the air, from one person to another. When someone infected with SARS-CoV-2 breathes, talks, sings, coughs, or sneezes, they release tiny virus-carrying particles from their nose and mouth. Another person nearby inhales those particles, and the virus latches onto cells lining the respiratory tract to start a new infection.
Airborne Particles Are the Main Route
The virus travels in two forms of airborne particles. Larger respiratory droplets, roughly the width of a human hair or bigger, are heavy enough to fall to the ground within about a meter (around three feet) of the person who released them. These are the particles generated most heavily by coughing, sneezing, and loud talking. If they land on your eyes, nose, or mouth, they can cause infection.
Smaller particles, sometimes called aerosols, are lighter and can float in the air for minutes to hours, drifting well beyond that one-meter range. This is why poorly ventilated indoor spaces pose the greatest risk. An analysis of more than 7,300 early COVID cases across 320 cities in China found that every identified cluster of three or more infections occurred indoors. Eighty percent of those clusters traced back to household settings, and 34% involved public transportation. Only a single outdoor transmission was recorded in the entire dataset.
Outdoors, moving air dilutes and disperses viral particles quickly. Indoors, they accumulate. The longer you share air with an infected person in a closed room, the higher your exposure.
Surface Contact Plays a Smaller Role
An infected person can deposit virus on surfaces by coughing into their hand and then touching a doorknob, countertop, or phone. If you touch that surface and then touch your face, the virus can potentially reach your respiratory tract. This route, called fomite transmission, was initially thought to be a major concern. It does happen, but researchers now consider it far less significant than breathing in contaminated air. Regular handwashing still reduces your risk, but improving ventilation matters more.
How the Virus Gets Inside Your Cells
Once viral particles reach the lining of your airways, the virus uses a spike-shaped protein on its surface to grab onto a receptor called ACE2 on the outside of your cells. Think of it like a key fitting into a lock. After the spike protein binds, the virus fuses with the cell membrane and injects its genetic material inside, hijacking the cell’s machinery to make copies of itself. Additional molecules on cell surfaces, including one called neuropilin-1, appear to help the virus latch on more efficiently, which partly explains why SARS-CoV-2 is so effective at establishing infection even when ACE2 levels in the airways are relatively low.
You Can Spread It Before You Feel Sick
One of the reasons COVID spread so widely is that people are contagious before they know they’re infected. A person typically becomes contagious about two days before symptoms appear. They remain contagious for 10 to 20 days after that, depending on immune function and how severe the illness becomes. People with mild cases tend to stop shedding significant amounts of virus sooner than those with serious infections.
A substantial portion of infected people never develop symptoms at all. Studies of the Omicron variant found asymptomatic rates ranging from 14% to 57% across different age groups. In children, the asymptomatic proportion has been measured as high as 60%. These individuals can still pass the virus to others, which makes COVID harder to contain through symptom-based screening alone.
Newer Variants Spread More Easily
The virus has evolved to become more transmissible over time. Contact tracing data from Spain during December 2021 compared the Omicron variant to its predecessor, Delta. Among household contacts of an infected person, 39% became infected with Omicron, compared to 26% with Delta. Vaccination status did not significantly change these numbers, meaning even vaccinated individuals transmitted the newer variant at higher rates. Each successive wave has generally featured variants with greater ability to spread, though severe illness rates have not tracked in the same direction.
What Reduces Transmission
Ventilation
Because airborne transmission dominates, improving indoor air quality is one of the most effective strategies. Opening windows, running exhaust fans, and using air filtration systems all reduce the concentration of viral particles in a room. Engineering guidelines recommend flushing enclosed spaces with at least three full air changes of clean air between occupied periods. In practical terms, any space where you can feel fresh air moving carries lower risk than a stuffy, sealed room.
Masks
Masks work by filtering out virus-carrying particles in both directions: protecting others from what you exhale, and protecting you from what you inhale. A study of Beijing households found that face masks reduced secondary transmission by 79% when all household members wore them before anyone showed symptoms. Another study found a 70% risk reduction for people who consistently wore masks when leaving home.
Filtration performance varies by material and fit. In controlled experiments, surgical masks filtered about 75% of very small particles (down to 0.02 micrometers) and 96% of larger bacterial-sized particles. Well-constructed cloth masks, particularly those using tightly woven cotton or layered fabrics like cotton combined with chiffon or silk, achieved filtration rates above 96% for particles larger than 0.3 micrometers. Loosely woven single-layer fabrics performed much worse, sometimes allowing over 90% of particles through. Fit matters as much as fabric: gaps around the nose and cheeks let unfiltered air bypass the mask entirely.
Distance and Duration
The closer you are to an infected person and the longer you spend near them, the more virus you’re likely to inhale. Keeping distance helps with larger droplets especially, since they fall to the ground within about a meter. For smaller aerosols that float further, distance alone is not enough in a poorly ventilated room, but it still reduces your dose. Brief encounters carry less risk than extended ones, which is why passing someone on a sidewalk is far less risky than sitting across from them at a two-hour dinner.