Coronaviruses are a large family of viruses that infect the respiratory tract, ranging from the common cold to severe diseases like COVID-19. The name comes from their appearance under an electron microscope: spherical particles covered in club-shaped spikes that resemble the sun’s corona. Seven coronaviruses are known to infect humans, four of which circulate widely and cause mild cold symptoms, while three have triggered serious outbreaks in the past two decades.
How the Virus Is Built
A coronavirus particle is tiny, roughly 80 to 160 nanometers across (thousands of times smaller than the width of a human hair). It’s wrapped in an unusually thick outer envelope, nearly twice the thickness of a typical biological membrane. Embedded in that envelope are spike proteins, large mushroom-shaped structures that jut outward and give the virus its crown-like look. These spikes are the virus’s primary tool for getting inside your cells.
Inside the envelope sits the virus’s genetic material: a single strand of RNA, which is among the largest genomes of any RNA virus. This genome carries the instructions the virus needs to hijack a cell’s machinery and make copies of itself.
How It Gets Into Your Cells
The spike protein works like a key. On the surface of many human cells, particularly in the lungs, throat, and nose, there’s a protein called ACE2 that acts as the lock. When a coronavirus spike latches onto ACE2, the virus can fuse with the cell membrane and inject its RNA inside. From there, the cell’s own machinery starts producing new virus particles instead of doing its normal job.
SARS-CoV-2, the virus behind COVID-19, has an extra trick that earlier coronaviruses lacked. A human enzyme called furin pre-activates the spike protein before it even reaches a target cell, making the entry process more efficient. This is one reason SARS-CoV-2 spreads so readily compared to its predecessor, the original SARS virus from 2003.
The Seven Human Coronaviruses
Four coronaviruses have circulated in humans since at least the 1960s: 229E, NL63, OC43, and HKU1. Most people encounter one or more of these during their lifetime. They typically cause mild upper-respiratory symptoms (stuffy nose, sore throat, low-grade fever) and resolve within a week or so.
The other three are far more dangerous:
- SARS-CoV caused Severe Acute Respiratory Syndrome in 2002-2003, with a case fatality rate around 10%. It was contained through quarantine measures and hasn’t been detected in humans since 2004.
- MERS-CoV emerged in 2012 in the Middle East, causing Middle East Respiratory Syndrome with a fatality rate of roughly 35%. It still circulates at low levels, primarily through contact with camels.
- SARS-CoV-2 appeared in late 2019 and caused the COVID-19 pandemic. Its pre-vaccination case fatality rate in the United States was about 1.7%, though this varied enormously by age: 24% in adults over 85 versus 0.01% in children aged 1 to 14.
How It Spreads
Coronaviruses spread primarily through respiratory droplets released when an infected person coughs, sneezes, talks, or breathes. Close contact (within about one meter) carries the highest risk, because larger droplets can land on your mouth, nose, or eyes. Smaller airborne particles can linger in poorly ventilated indoor spaces, extending the risk beyond arm’s length.
The virus can also survive briefly on surfaces. Touching a contaminated doorknob or phone and then touching your face is a secondary route, though it’s considered less common than direct respiratory transmission.
Symptoms and Incubation
After exposure to SARS-CoV-2, symptoms typically appear within 3 to 6 days, though the window can stretch to 14 days in some cases. Current Omicron-related variants tend to have a shorter incubation period than the original 2020 strains.
Common symptoms include fever or chills, cough, sore throat, congestion or runny nose, fatigue, muscle aches, and headache. Some people experience loss of taste or smell, shortness of breath, nausea, vomiting, or diarrhea. Symptoms can shift with new variants and may differ depending on whether you’ve been vaccinated or previously infected. Many infections, especially in vaccinated individuals, feel similar to a bad cold and resolve within about 10 days.
Testing for Coronavirus
Two main types of tests detect an active SARS-CoV-2 infection. Nucleic acid tests (commonly called PCR tests) are the most accurate. They detect viral genetic material and are highly sensitive, meaning they catch infections even when viral levels are low.
Antigen tests, including the rapid at-home kits most people are familiar with, are also highly specific (meaning a positive result is reliable) but less sensitive than PCR. A single negative rapid test doesn’t rule out infection. If you have symptoms and test negative, the FDA recommends repeating the test up to three times, spacing each test 48 hours apart.
How Vaccines Work Against It
All current COVID-19 vaccines target the spike protein, since that’s what the virus uses to enter cells. By teaching your immune system to recognize the spike, vaccines prepare your body to neutralize the virus before it can establish a serious infection.
mRNA vaccines (like those from Pfizer-BioNTech and Moderna) deliver a small piece of genetic code into your muscle cells. Your cells use those instructions to build a harmless fragment of the spike protein, display it on their surface, and trigger an immune response. The mRNA itself is broken down and cleared from the body shortly after. Protein subunit vaccines (like the Novavax option) take a more traditional approach, delivering pre-made spike protein fragments along with an adjuvant, an ingredient that amplifies the immune response. Both approaches leave your body with memory immune cells, T-lymphocytes and B-lymphocytes, that can mount a faster defense if you encounter the actual virus.
Long-Term Effects After Infection
Some people develop lingering symptoms after their initial COVID-19 illness, a condition the WHO calls post-COVID-19 condition, widely known as long COVID. It’s defined by symptoms that begin within three months of infection and persist for at least two months. Over 200 different symptoms have been reported, but the most common ones are fatigue, muscle or joint pain, breathlessness, headaches, difficulty concentrating (often called “brain fog”), and changes in taste. Sleep problems, depression, and anxiety also occur frequently.
Long COVID can affect people regardless of how severe their initial infection was, including those who had mild or even asymptomatic cases. The risk appears to be lower in vaccinated individuals, though it’s not eliminated entirely.