COVID-19 is an infectious respiratory disease caused by the virus SARS-CoV-2. First identified in late 2019, it spread globally and has since become an endemic virus that continues to circulate, causing seasonal waves of infection. The disease ranges from completely asymptomatic to life-threatening, depending on a person’s age, immune status, and underlying health.
How the Virus Enters Your Body
SARS-CoV-2 is covered in spike proteins that latch onto a specific receptor called ACE2, found on the surface of human cells. Think of ACE2 as a lock and the spike protein as a key. Once the spike protein binds to ACE2, an enzyme on the cell’s surface helps the virus fuse with the cell membrane and slip inside. From there, the virus hijacks the cell’s machinery to make copies of itself.
The virus typically enters through the mucous membranes in the nose and throat. It can also travel directly into the lower airways, infecting the cells lining the bronchial tubes and the tiny air sacs deep in the lungs. ACE2 receptors are especially abundant in those deep lung cells, which helps explain why COVID-19 so often becomes a lower respiratory illness. But ACE2 receptors also exist in the heart, kidneys, intestines, and blood vessels, which is why the virus can affect far more than just the lungs.
How COVID-19 Spreads
The virus spreads primarily through respiratory droplets released when an infected person coughs, sneezes, talks, or breathes. Close contact, generally within about one meter, carries the highest risk. You can also pick up the virus by touching a contaminated surface and then touching your mouth, nose, or eyes, though this is a less common route.
In poorly ventilated indoor spaces, smaller airborne particles can linger and travel farther than one meter, increasing the chance of infection even without close face-to-face contact. This is especially relevant in crowded rooms with limited airflow. Improving ventilation, whether by opening windows or using air filtration, reduces the concentration of virus particles in shared indoor spaces.
Common Symptoms
Fever, cough, and fatigue are the three most common symptoms. Many people also experience muscle weakness, chest tightness, and sore throat. A sudden loss of smell or taste, even without nasal congestion, is one of the more distinctive signs of COVID-19. Roughly 80% of infected people who don’t have a stuffy nose still report a reduced or absent sense of smell.
Symptoms typically appear two to five days after exposure, though the window can stretch up to 14 days. Most people develop mild to moderate illness that resolves on its own within one to two weeks. Some people, particularly younger and healthier individuals, never develop symptoms at all but can still spread the virus to others.
When It Becomes Serious
In a smaller percentage of cases, COVID-19 progresses to pneumonia, where inflammation fills the air sacs in the lungs with fluid and makes it difficult to get enough oxygen. This typically happens during the second week of illness, often after an initial period of improvement. The body’s immune response can overshoot, triggering a flood of inflammatory signals that damages not just the lungs but other organs as well.
The heart is vulnerable because it has a high concentration of ACE2 receptors. COVID-19 can cause inflammation of the heart muscle, irregular heartbeats, and in some cases reduced pumping ability. The kidneys can also sustain direct damage from the virus or suffer as a secondary consequence of widespread inflammation and low oxygen levels. This chain reaction between damaged lungs and stressed kidneys can accelerate organ failure in the most severe cases.
Who Faces the Highest Risk
Certain groups are significantly more likely to develop severe illness. According to the CDC, the conditions that raise your risk include:
- Chronic lung disease such as asthma or COPD
- Heart conditions including heart failure and coronary artery disease
- Diabetes (type 1 or type 2)
- Chronic kidney disease at any stage
- Obesity
- Weakened immune system from medications, organ transplant, or conditions like HIV
- Cancer
- Neurological conditions including dementia and stroke history
- Pregnancy
Age remains one of the strongest predictors. Older adults, particularly those over 65, face a much higher risk of hospitalization and death compared with younger people, even without other medical conditions.
Testing: Rapid vs. PCR
Two main types of tests detect active COVID-19 infection. PCR tests, which are processed in a lab, remain the gold standard. They detect tiny amounts of viral genetic material and are highly accurate. The trade-off is turnaround time, which can range from several hours to a couple of days.
Rapid antigen tests give results in about 15 minutes but are less sensitive. In studies comparing the two, rapid tests correctly identified about 72% of infections in people with symptoms and only about 61% in people without symptoms. However, when someone has a high amount of virus in their system, rapid test accuracy climbs above 90%. This means a negative rapid test doesn’t fully rule out infection, especially in the first day or two of symptoms. If you test negative but feel sick, testing again 24 to 48 hours later improves your chances of getting an accurate result, since viral levels rise over that window.
Treatment Options
For most people with mild illness, rest, fluids, and over-the-counter fever reducers are sufficient. But for those at higher risk of severe disease, antiviral medications can prevent progression if started early, typically within the first five days of symptoms.
The most widely used outpatient antiviral is Paxlovid, an oral medication taken twice daily for five days. Remdesivir, given as an infusion, is approved for both outpatients and hospitalized patients, including children. Molnupiravir is another oral option for adults when other treatments aren’t suitable. All three work by interfering with the virus’s ability to replicate.
For patients sick enough to be hospitalized, treatment shifts toward managing the immune system’s overreaction. Immune-modulating drugs help tamp down the dangerous inflammatory cascade that drives organ damage in severe cases. These are used alongside oxygen support and, when needed, mechanical ventilation.
Vaccine Protection
Updated COVID-19 vaccines are reformulated periodically to match circulating variants, similar to the annual flu shot. Based on CDC data from the 2024-2025 season, the updated vaccine reduced COVID-related emergency department and urgent care visits by about 33% in adults. Among adults 65 and older with healthy immune systems, protection against hospitalization was stronger, at 45 to 46%. For older adults with weakened immune systems, that figure was around 40%.
Protection against the most severe outcomes, including ICU admission and death, has historically been higher and more durable than protection against milder illness. The vaccines don’t eliminate your chance of catching COVID-19, but they substantially lower the odds that an infection will put you in the hospital.
Long COVID
Some people experience symptoms that persist for months or even years after their initial infection. This condition, known as Long COVID, has been associated with more than 200 different symptoms, though a few stand out as the most common: persistent fatigue, brain fog (difficulty concentrating or thinking clearly), and post-exertional malaise, where symptoms flare after physical or mental effort that previously would have been manageable.
Other frequently reported issues include shortness of breath, heart palpitations, headaches, dizziness upon standing, sleep disturbances, joint and muscle pain, digestive problems, and changes in mood. Symptoms can come and go unpredictably, improving for weeks before returning. Long COVID ranges from mildly annoying to disabling, and it can develop even after a mild initial infection. The condition sometimes requires care from multiple specialists because it affects so many different body systems simultaneously.