SARS-CoV-2 is the virus that causes COVID-19. The name stands for Severe Acute Respiratory Syndrome Coronavirus 2, and it belongs to a family of viruses called coronaviruses, named for the crown-like spikes covering their surface. First identified in late 2019, SARS-CoV-2 triggered a global pandemic that the World Health Organization maintained as a public health emergency until May 2023. The virus continues to circulate worldwide, though most people now have some degree of immunity from prior infection, vaccination, or both.
What the Virus Looks Like
SARS-CoV-2 is a spherical particle roughly 80 to 160 nanometers across, far too small to see without an electron microscope. It’s wrapped in an unusually thick fatty envelope, nearly twice as thick as a typical biological membrane, packed with a network of proteins that give it structural stability. Sitting on that envelope are three types of proteins, the most important being the spike protein: the large, club-shaped projections that give coronaviruses their distinctive appearance.
Inside the envelope, the virus carries a single strand of RNA, its genetic blueprint. At 30,000 genetic letters long, it’s one of the largest RNA genomes of any virus. That RNA is bundled with a structural protein called nucleoprotein, forming a coiled package that carries everything the virus needs to hijack a human cell and make copies of itself.
How It Gets Into Your Cells
The spike protein is the virus’s key tool for infection. Each SARS-CoV-2 particle carries spikes arranged in groups of three. One spike in each trio can flip upward into an “up” position, exposing a region that locks onto a receptor called ACE2, found on the surface of cells in your airways, lungs, heart, gut, and other organs. When the spike is in the “down” position, it can’t attach, which is one reason not every encounter with the virus leads to infection.
The binding process works like a two-step clamp. One loop on the spike acts as an anchor, making initial contact with the ACE2 receptor. A second loop swings in from the opposite side and locks the connection in place. Once both ends are secured, additional charged sections of the spike pull tight against the cell surface, reinforcing the bond. After this firm attachment, the virus fuses with the cell membrane and releases its RNA inside, commandeering the cell’s own machinery to produce thousands of new viral copies.
How It Spreads
SARS-CoV-2 spreads primarily from person to person through respiratory droplets and smaller airborne particles called aerosols. When someone who is infected coughs, sneezes, talks, or sings, they release droplets of varying sizes. Larger droplets (bigger than 5 to 10 micrometers) tend to fall to the ground within about one meter, which is why close contact carries the highest risk. Smaller aerosol particles can linger in the air longer, especially in poorly ventilated indoor spaces.
The virus can also land on surfaces, creating what’s known as fomites. Touching a contaminated surface and then touching your eyes, nose, or mouth is a possible but less common route of infection. The dominant pathway remains breathing in virus-laden particles released by someone nearby.
Symptoms and Incubation Period
The time between exposure and symptom onset has shortened as the virus has evolved. Early strains had an average incubation period of about 6.5 days. The Delta variant shortened that to roughly 4.3 days, and Omicron variants brought it down further to a median of 3 to 4 days.
Most people experience respiratory symptoms: cough, shortness of breath, sore throat, and congestion. Fatigue and “brain fog” are the most commonly reported complaints, including among people whose initial illness was mild. Brain fog can involve slower thinking, difficulty concentrating, and trouble with memory and attention. Some people develop gastrointestinal problems. Others lose their sense of smell.
Less commonly, the virus can trigger serious neurological effects. Early in the pandemic, doctors reported large, severe strokes in patients who were younger than the typical stroke population. Ongoing research continues to investigate links between SARS-CoV-2 infection and headaches, sleep disturbances, persistent fatigue, nerve damage, and dysfunction of the autonomic nervous system (the network that controls heart rate, digestion, and other involuntary functions). Some people experience these symptoms for weeks or months after the initial infection, a condition widely known as long COVID.
Testing: PCR vs. Rapid Antigen
Two main types of tests detect SARS-CoV-2. PCR tests (the kind processed in a lab) remain the most sensitive, picking up even small amounts of viral genetic material. Rapid antigen tests, the at-home kind that deliver results in 15 minutes, are less sensitive overall but perform significantly better when you’re symptomatic.
In a CDC study from 2022 to 2023, rapid antigen tests detected the virus 56% of the time in people experiencing symptoms on the day of testing, compared to PCR as the benchmark. On days when fever was present, sensitivity jumped to 77%. But on days with no symptoms, sensitivity dropped to just 18%. The practical takeaway: if you feel sick and a rapid test is negative, testing again 24 to 48 hours later (or getting a PCR test) gives you a more reliable answer. A single negative rapid test on a day you feel fine is not especially reassuring.
How Vaccines Work Against It
The mRNA vaccines developed for COVID-19 target the spike protein. They deliver a small piece of lab-made genetic instructions into your muscle cells, which then produce a harmless fragment of the spike protein. Your cells display that fragment on their surface, and your immune system recognizes it as foreign. This triggers the production of antibodies and activates immune cells that are primed to fight the real virus if you encounter it later. The mRNA itself is broken down and cleared from the body shortly after it does its job.
Because the spike protein is what the virus uses to enter your cells, antibodies that target it can block infection or reduce its severity. As the virus mutates and the spike protein changes shape, updated vaccine formulations are released to better match circulating variants.
Where Things Stand Now
The WHO declared the COVID-19 public health emergency of international concern over in May 2023, but the virus hasn’t disappeared. SARS-CoV-2 continues to circulate, mutate, and cause seasonal waves of illness. More than 110 countries have strengthened genomic surveillance systems to track new variants as they emerge. The virus is now treated as an ongoing respiratory pathogen, similar in public health terms to influenza, requiring continued monitoring, updated vaccines, and awareness of its potential to cause serious illness in vulnerable populations.