You get coronavirus primarily by breathing in tiny virus-carrying particles that an infected person releases when they breathe, talk, cough, or sneeze. The virus enters your body through your nose, mouth, or (less commonly) your eyes. Most transmission happens indoors, at close range, and in poorly ventilated spaces, though the virus can also travel across a room in fine aerosol particles that linger in the air.
Breathing It In: The Main Route
SARS-CoV-2 spreads through two types of respiratory particles. Larger droplets (bigger than 5 to 10 micrometers across) are released when someone coughs or sneezes and tend to fall to the ground within about one meter, roughly three feet. Smaller particles, sometimes called aerosols, are less than 5 micrometers across. These are light enough to float in the air for minutes to hours and can travel well beyond arm’s length, especially in enclosed rooms with poor airflow.
This is why close, face-to-face contact with an infected person carries the highest risk. But it also explains outbreaks in restaurants, choir rehearsals, and office buildings where people were sitting far apart. In stagnant indoor air, aerosols accumulate the way cigarette smoke would. The estimated number of viral particles needed to start an infection is remarkably small: research based on primate studies suggests somewhere between 36 and 179 individual virus particles inhaled into the lungs may be enough.
You Don’t Have to Be Near a Sick Person
Not everyone who spreads the virus looks or feels sick. A meta-analysis in the Journal of the Association of Medical Microbiology and Infectious Disease Canada estimated that about 17% of all COVID infections are fully asymptomatic, meaning the person never develops symptoms at all. That figure ranged from 4% to 41% depending on the setting, with higher rates in aged-care facilities.
People without symptoms are less contagious than those who are actively coughing and sneezing. Their transmission risk is roughly 42% lower. But because they feel fine and go about their normal routines, they still contribute meaningfully to spread. On top of that, people who do eventually develop symptoms are often most contagious in the one to two days before those symptoms appear, a period when they have no reason to suspect they’re infected.
Incubation Period: Exposure to Symptoms
The time between catching the virus and feeling sick has shortened as the virus has evolved. Early in the pandemic, the average incubation period was about 6.5 days. With the Delta variant, it dropped to around 4.3 days. Omicron-lineage variants, which have dominated since late 2021, typically show symptoms within 3 to 4 days of exposure. This faster onset also means faster spread through households and workplaces, since each link in a chain of transmission takes less time.
Surfaces and Hands
Early in the pandemic, there was significant concern about catching COVID from contaminated surfaces like doorknobs, packages, and grocery carts. The virus can survive on smooth surfaces: lab studies found it remained detectable on plastic and stainless steel for up to seven days, and on glass and banknotes for up to four days, under controlled conditions. In real-world settings with fluctuating temperature, humidity, and UV light, survival times are shorter.
That said, surface transmission is now considered a minor route compared to airborne spread. The virus would need to transfer from a surface to your hand and then from your hand to your nose, mouth, or eyes in sufficient quantity to cause infection. It can happen, but it accounts for a small fraction of cases. Regular handwashing still reduces your risk, just not as much as improving the air you breathe.
Transmission Through the Eyes
Your eyes are a less common but real entry point. The surface of the eye connects to the upper respiratory tract through the tear ducts, giving the virus a direct path to your nose and throat. In one study of 535 COVID patients, 5% had conjunctival congestion (red, irritated eyes), and four of those patients had it as their very first symptom. The eye’s natural drainage system may actually help the virus along by washing it from the tear film down through the nasolacrimal duct into the nasal passages. This is why touching your eyes with unwashed hands increases risk, and why healthcare workers in high-exposure settings wear eye protection.
Animals to Humans
Animal-to-human transmission is rare but has been documented. Farmed mink in the Netherlands, Denmark, Poland, and the United States transmitted the virus back to workers, and genetic analysis confirmed unique mink-related mutations in some human cases. White-tailed deer in Canada, pet hamsters in Hong Kong, and at least one cat in Thailand have also been linked to human infections through close contact. For most people, this isn’t a practical concern, but it’s worth knowing that the virus circulates in some animal populations and can occasionally jump back.
Why Indoor Air Matters So Much
Ventilation is one of the strongest predictors of transmission risk in any shared indoor space. The CDC recommends indoor air be completely replaced at least five times per hour. For context, the average American home manages fewer than 0.5 air changes per hour, roughly one-tenth of that target. This gap explains why homes, apartments, and poorly ventilated buildings are where so much transmission occurs.
Opening windows, using portable air purifiers with HEPA filters, or upgrading HVAC filters to MERV-13 ratings all increase the rate at which virus-laden air gets diluted or cleaned. Outdoors, air disperses so quickly that transmission risk drops dramatically, which is why outdoor gatherings have consistently been safer than indoor ones throughout the pandemic.
How Masks Reduce Your Risk
Masks work by filtering the particles you inhale and reducing the particles you exhale. But the type of mask matters considerably. A CDC study of 534 people found that N95 or KN95 respirators reduced the odds of testing positive by 83% compared to wearing no mask in indoor public settings. Surgical masks reduced the odds by 66%. Cloth masks showed a 56% reduction, though that result wasn’t statistically significant, meaning it could have been due to chance.
Fit matters as much as material. An N95 that gaps at the sides performs closer to a surgical mask. If you’re in a high-risk indoor setting, a well-fitting N95 or KN95 remains the most effective option available outside of avoiding the space entirely.
Situations With the Highest Risk
Putting all of this together, the riskiest scenarios share a few features: indoor spaces, poor ventilation, close proximity, long duration, and loud talking or singing (which produces more aerosol). A crowded bar with no open windows is far riskier than a brief trip through a well-ventilated grocery store. Household transmission remains one of the most common settings because you share air with the same people for hours in relatively small rooms, often with limited ventilation.
Your personal risk in any given situation depends on how much virus is in the air you’re breathing and how long you’re breathing it. Anything that reduces viral concentration (ventilation, filtration, masks, distance) or shortens your exposure time lowers your chances of inhaling enough particles to get infected.