COVID-19 is an infectious disease that has had a profound impact across the globe. While caused by a virus, understanding the “causes” of COVID-19 means looking beyond the pathogen itself. The spread and severity of the illness are driven by a combination of factors. These include how the virus travels from person to person, the environments where it thrives, and the unique characteristics of each individual who becomes exposed.
The Primary Cause: The SARS-CoV-2 Virus
The disease known as COVID-19 is directly caused by a pathogen named Severe Acute Respiratory Syndrome Coronavirus 2, or SARS-CoV-2. This virus is part of a large family of coronaviruses, named for the crown-like spikes on their surface. These spike proteins play a direct role in how the virus infects the human body.
SARS-CoV-2 uses its spike proteins to attach to specific receptors on the surface of human cells, most notably the Angiotensin-Converting Enzyme 2 (ACE2) receptor. These receptors are abundant in the respiratory tract, which is why the virus so effectively targets the lungs. Once attached, the virus can enter the cell, hijack its machinery to replicate, and create new viral particles that can then infect other cells or be transmitted to other people.
Airborne Transmission
The primary method by which SARS-CoV-2 moves between people is through the air. When an infected person breathes, talks, coughs, or sneezes, they release respiratory fluids containing the virus. These fluids manifest as particles of various sizes that influence transmission.
Larger particles, often referred to as droplets, are heavier and fall out of the air relatively quickly, landing within a few feet of the person who expelled them. Infection can occur if these droplets land on the eyes, nose, or mouth of a nearby individual. Smaller, lighter particles, known as aerosols, behave differently and can remain suspended in the air for minutes to hours, traveling well beyond the immediate vicinity of the infected person.
Inhalation of these aerosolized viral particles is a major route of transmission. This means a person does not need to be in direct, face-to-face contact with an infected individual to be at risk, especially in indoor settings. The act of breathing alone is enough to generate these aerosols, meaning that even people with no symptoms can unknowingly spread the virus.
Exposure Proximity and Duration
The risk of contracting COVID-19 is heavily influenced by the closeness of contact and the length of time spent with an infected individual. These two factors—proximity and duration—work together to determine the likelihood of transmission. Simply passing by someone with the virus carries a different level of risk compared to sharing an office with them for an entire day.
Closer physical distance to an infected person significantly increases the probability of inhaling a sufficient quantity of viral particles. The duration of the exposure is also a determining factor. Prolonged contact, such as spending 15 minutes or more in close conversation, increases the cumulative dose of virus a person is exposed to, which can overwhelm the body’s initial immune defenses.
Compounding Environmental Factors
The environment where exposure occurs significantly impacts the probability of transmission. Physical spaces can either help dissipate the virus or allow it to accumulate to infectious concentrations. These conditions can lead to super-spreader events, where one person infects many others.
Ventilation is a primary environmental factor. Outdoors or in well-ventilated indoor spaces, infectious aerosols are quickly dispersed, lowering the virus concentration in the air and reducing risk. In poorly ventilated indoor areas, these viral particles build up over time, increasing the likelihood that anyone present will inhale an infectious dose.
Crowding is another major consideration. Densely populated indoor spaces increase risk because there are more potential sources of the virus and more targets for infection. This is why places like crowded bars, restaurants, and conference halls have been identified as high-risk. The combination of poor ventilation and high occupancy creates an environment where the virus can spread with high efficiency.
Individual Susceptibility and Risk Factors
While external factors determine exposure, the outcome depends on the individual. Not everyone exposed to SARS-CoV-2 will become infected, and not all who are infected will experience the same illness severity. A person’s unique biological makeup and health status play a large part in their susceptibility to infection and severe disease.
Vaccination status is a primary determinant of disease outcome. COVID-19 vaccines prepare the immune system to fight the virus, significantly reducing the risk of severe illness, hospitalization, and death. While they may not always prevent infection entirely, they are highly effective at preventing the most serious consequences of the disease.
Advanced age is another risk factor, with older adults being more likely to develop severe symptoms. The immune system’s ability to respond to new pathogens tends to diminish with age, making it harder for the body to control the viral infection. This can lead to more severe outcomes, including pneumonia and respiratory failure.
Underlying medical conditions also increase the risk of severe COVID-19. These conditions can impair the body’s ability to handle the physiological stress of a viral infection, making it more probable that an infection will progress to a severe or life-threatening stage. Vulnerable individuals include those with:
- Chronic lung disease
- Serious heart conditions
- Diabetes
- Obesity
- Conditions that weaken the immune system