COVID-19 is a respiratory illness caused by a virus called SARS-CoV-2, a type of coronavirus. First identified in late 2019, it spread globally and remains in circulation today. The virus primarily attacks the respiratory system but can affect multiple organs, and its severity ranges from no symptoms at all to life-threatening illness.
How the Virus Infects Your Body
SARS-CoV-2 is a spherical virus covered in protein spikes that give it a crown-like appearance under a microscope (corona means “crown” in Latin). Those spike proteins are the key to how it gets inside your cells. Each spike locks onto a specific receptor found on the surface of human cells, particularly in the lungs, nose, and throat. Once attached, the spike changes shape, allowing the virus to fuse with the cell membrane and slip inside. From there, it hijacks the cell’s machinery to make copies of itself, which then spread to infect more cells.
This receptor is also found in the heart, kidneys, intestines, and blood vessels, which helps explain why COVID-19 can cause problems well beyond the lungs.
How COVID-19 Spreads
The virus spreads mainly through the air. When an infected person breathes, talks, coughs, or sneezes, they release droplets and tiny particles containing the virus. Other people can inhale those particles, or the particles can land on someone’s eyes, nose, or mouth. In some cases, contaminated surfaces play a role, though airborne transmission is the dominant route.
One important detail: anyone infected with COVID-19 can spread it even if they have no symptoms at all. This made the virus especially difficult to contain during the pandemic, since people could unknowingly pass it to others before they ever felt sick.
Incubation Period
The time between exposure and symptom onset has shortened as the virus has evolved. Early in the pandemic, the average incubation period was about 6.5 days. With the Delta variant, that dropped to roughly 4.3 days. With Omicron and its subvariants, most people develop symptoms within 3 to 4 days of exposure.
Symptoms to Watch For
COVID-19 symptoms overlap significantly with the flu and common cold, which is one reason testing matters. The most commonly reported symptoms include:
- Fever or chills
- Cough
- Shortness of breath
- Sore throat
- Congestion or runny nose
- New loss of taste or smell
- Fatigue
- Muscle or body aches
- Headache
- Nausea, vomiting, or diarrhea
Most people experience mild to moderate illness and recover at home. However, some cases turn severe. Emergency warning signs include trouble breathing, persistent chest pain or pressure, new confusion, inability to stay awake, and pale, gray, or blue-tinted skin, lips, or nail beds. These warrant a call to 911.
Testing: Rapid vs. PCR
Two main types of tests are used to detect an active COVID-19 infection. PCR tests, processed in a lab, are the more accurate option. Rapid antigen tests, the kind you can take at home, are convenient but less sensitive, especially early in the infection.
In a study of 225 adults and children with confirmed infections, rapid antigen tests correctly identified the virus 64% of the time compared with same-day PCR results. Sensitivity peaked at 77% around day four of illness. For people without symptoms, sensitivity dropped to just 20%, meaning rapid tests miss the majority of asymptomatic infections.
If you’re using a home test, taking two tests spaced 48 hours apart significantly improves accuracy compared to a single test, boosting peak sensitivity to about 85%. This is especially useful in the first few days after symptoms appear, when a single test is most likely to give a false negative.
Treatment Options
For people with mild to moderate COVID-19 who are at higher risk of getting sicker, antiviral medications can reduce the chance of hospitalization. The most widely used is Paxlovid, an oral antiviral approved for adults and authorized for adolescents, typically started within five days of symptom onset. Remdesivir, given by infusion, is another option approved for both adults and children.
For hospitalized patients, treatment shifts toward managing the body’s inflammatory response. Several immune-modulating drugs are approved or authorized for use in severe cases, helping to calm the overactive immune reaction that can damage the lungs and other organs. The specific treatment depends on how sick the patient is and which organ systems are affected.
Most people with mild illness don’t need prescription treatment. Rest, fluids, and over-the-counter fever and pain relievers are typically enough.
Long COVID
A significant percentage of people continue to experience symptoms long after the initial infection clears. Research pooling data from multiple studies found that roughly 27 to 30% of people had lingering symptoms, whether measured at less than six months or six months and beyond after infection. Common long COVID symptoms include persistent fatigue, brain fog, shortness of breath, and joint or muscle pain.
Long COVID prevalence appears to vary by which variant caused the infection and when during the pandemic someone was infected. It can affect anyone, including people who had a mild initial illness, though severe initial infections carry higher risk.
Vaccination
COVID-19 vaccines are updated regularly to match circulating variants, similar to the annual flu shot. The 2025–2026 vaccine is recommended for everyone ages 6 months and older, with the strongest emphasis on people at increased risk for severe disease. This includes adults 65 and older, people with chronic health conditions, healthcare workers, and those living in long-term care facilities.
For adults 65 and older, the current guidance recommends two doses spaced about six months apart, regardless of previous vaccination history. For people ages 6 months through 64, vaccination is recommended through shared decision-making with a healthcare provider, weighing individual risk factors. People who are moderately or severely immunocompromised follow a separate, more intensive schedule.
You don’t need to bring documentation of a qualifying condition. The CDC allows self-attestation, meaning you can simply state that you have a risk factor and receive the vaccine without proof.
Who Is Most at Risk
While COVID-19 can affect anyone, certain groups face a higher chance of severe illness. Adults 65 and older remain the most vulnerable age group. Other risk factors include heart disease, diabetes, chronic lung conditions, obesity, weakened immune systems (from medications or conditions like cancer), and pregnancy. People in congregate living settings, such as nursing homes, face both higher exposure risk and higher risk of complications.
Early in the pandemic, before widespread immunity from vaccines and prior infections, the global infection fatality rate was estimated at roughly 0.9%, meaning about 9 out of every 1,000 infected people died. That rate has dropped considerably as population immunity has built up, but COVID-19 continues to cause hospitalizations and deaths, particularly among older and immunocompromised individuals.