Coronaviruses are a large family of viruses that infect the respiratory tract of humans and animals, ranging from the cause of many common colds to the virus behind the COVID-19 pandemic. Seven coronaviruses are known to infect people. Four of them circulate widely every year and typically cause mild cold symptoms, while three others, SARS, MERS, and SARS-CoV-2, have caused serious outbreaks with significantly higher rates of severe illness.
Structure of a Coronavirus
Coronaviruses get their name from the crown-like ring of spikes visible under an electron microscope. Those spikes are one of four structural proteins that make up the virus. The spike protein is what latches onto human cells to start an infection. A membrane protein and an envelope protein form the outer shell, while a nucleoprotein inside packages and protects the virus’s genetic material.
That genetic material is a single strand of RNA roughly 30,000 genetic letters long, making coronaviruses the largest of all RNA viruses. This matters because larger genomes are harder to copy accurately. To compensate, coronaviruses carry a built-in proofreading tool that checks for errors during replication. This proofreader makes their copying accuracy about 24 times higher than influenza viruses, which is one reason flu mutates and drifts so rapidly by comparison.
The Four Common Cold Coronaviruses
Most people have been infected by a coronavirus without knowing it. Four types, known as 229E, NL63, OC43, and HKU1, circulate regularly in the human population and cause mild to moderate upper respiratory symptoms: runny nose, sore throat, cough, and occasionally a low fever. In the United States, infections with these viruses peak in fall and winter, though they can occur any time of year. For most healthy adults, these infections are indistinguishable from a cold caused by a rhinovirus or any other common respiratory pathogen.
Three Coronaviruses That Caused Major Outbreaks
The three coronaviruses that jumped from animals to humans more recently have been far more dangerous. SARS (Severe Acute Respiratory Syndrome) emerged in 2002 and had a case fatality rate of about 9.5%. MERS (Middle East Respiratory Syndrome), identified in 2012, was deadlier still, killing roughly 34.4% of confirmed cases. COVID-19, caused by SARS-CoV-2, had a lower case fatality rate of around 2.1% but spread far more efficiently, ultimately infecting hundreds of millions of people worldwide.
All three of these viruses are betacoronaviruses, one of four genera in the coronavirus family. The other genera are alphacoronaviruses (which include two of the common cold strains, 229E and NL63), gammacoronaviruses, and deltacoronaviruses. Gamma and delta strains primarily infect birds and pigs rather than people.
Where Coronaviruses Come From
Bats are the natural reservoir for most coronaviruses. Horseshoe bats in particular harbor a wide variety of these viruses, and investigations have traced the origins of SARS back to bat populations in China. The virus didn’t jump directly from bats to humans, though. It passed through an intermediate animal host first. For SARS, civet cats sold in live animal markets served as the bridge species. For MERS, dromedary camels are the ongoing source of human infections.
This pattern of moving through an intermediate animal before reaching people is common for coronaviruses. It gives the virus an opportunity to adapt to mammalian cells in a species that has closer contact with humans than wild bats typically do.
How Coronaviruses Infect Cells
Infection starts when the spike protein on the virus’s surface locks onto a receptor on a human cell. For SARS and SARS-CoV-2, that receptor is ACE2, a protein found on cells lining the airways, lungs, and other organs. The spike protein exists in a closed form that hides its binding site and an open form that exposes it. Once the spike opens and grabs ACE2, the attachment triggers a chain of structural changes. The spike splits apart, and a section called the fusion peptide punches into the cell membrane, merging the virus’s outer layer with the cell. This releases the viral RNA inside, where the cell’s own machinery begins producing copies of the virus.
The specific receptor a coronavirus targets determines which animals it can infect and which tissues in the body it reaches. MERS uses a different receptor than SARS, which partly explains why the two diseases affect the lungs differently despite both being betacoronaviruses.
How Coronaviruses Spread
Human coronaviruses spread primarily through respiratory droplets produced when an infected person coughs, sneezes, talks, or breathes. Close contact is the main driver of transmission. Larger droplets tend to fall within a few feet, but smaller particles can linger in the air longer, especially in poorly ventilated indoor spaces. Contact transmission is also possible: touching a surface contaminated with virus and then touching your face can introduce the virus to your airways, though this appears to be a less common route than direct respiratory exposure.
Testing for Coronavirus Infections
Two main types of tests detect active coronavirus infections. PCR tests (a type of nucleic acid test) look for the virus’s genetic material and are highly sensitive, catching infections even when viral levels are low. The tradeoff is cost, typically $75 to $100, and turnaround time, usually one to three days for lab-based versions. One important quirk: viral RNA can remain detectable in the body for up to 90 days after infection, so a positive PCR test doesn’t always mean you’re currently contagious.
Antigen tests detect viral proteins rather than genetic material. They’re cheaper ($5 to $50), return results in 15 to 30 minutes, and are widely available for home use. They’re less sensitive than PCR tests, meaning they miss more infections, particularly in people without symptoms. A single negative antigen test isn’t fully reliable. Repeating the test two more times, spaced 48 hours apart, gives a much more accurate picture.
How Coronaviruses Are Treated
For the four common cold coronaviruses, treatment is the same as any mild respiratory infection: rest, fluids, and over-the-counter symptom relief. The serious coronaviruses have driven the development of targeted antiviral drugs, particularly since the COVID-19 pandemic. These drugs work by blocking key enzymes the virus needs to replicate. One class targets the virus’s main protease, an enzyme that cuts newly made viral proteins into functional pieces. Without this cutting step, the virus can’t assemble new copies of itself. Another class interferes with the enzyme that copies the virus’s RNA, slowing replication directly. Both approaches aim to reduce the amount of virus in the body early in infection, before the illness progresses to its most dangerous phase.
Vaccines have been the other major tool against severe coronavirus disease. The rapid development of COVID-19 vaccines, many of which target the spike protein to train the immune system to recognize and neutralize the virus, represented an unprecedented response to a coronavirus outbreak. No widely used vaccines exist for SARS or MERS, largely because SARS disappeared before vaccine trials could be completed and MERS cases remain relatively rare and geographically limited.