Coronaviruses are a family of viruses that infect the respiratory tract, ranging from the cause of many common colds to the source of severe outbreaks like COVID-19. Seven coronaviruses are known to infect humans, and most people will catch at least one of them during their lifetime.
The Seven Human Coronaviruses
Of the seven coronaviruses that circulate in human populations, four cause everyday illness and three have triggered serious outbreaks. The everyday strains, known as 229E, NL63, OC43, and HKU1, typically produce mild to moderate upper respiratory infections indistinguishable from a regular cold: runny nose, sore throat, headache, cough, and fever. These viruses circulate year-round, though they peak in fall and winter, and nearly everyone encounters them at some point.
The three remaining strains are more dangerous. SARS-CoV caused an epidemic of severe acute respiratory syndrome from 2002 to 2004, with a case fatality rate of about 10% across roughly 8,100 cases. MERS-CoV, first identified in 2012, causes Middle East respiratory syndrome and kills about 34% of those diagnosed, though it spreads far less easily between people (fewer than 2,500 confirmed cases between 2012 and 2019). SARS-CoV-2, the virus behind COVID-19, emerged in late 2019 and spread globally. Its fatality rate in reported cases was initially estimated around 2%, far lower per case than SARS or MERS, but its extreme transmissibility meant it killed far more people overall.
All seven belong to two subgroups. The alpha coronaviruses include 229E and NL63. The beta coronaviruses include OC43 and HKU1, plus all three of the severe strains.
What a Coronavirus Looks Like
Coronaviruses get their name from the Latin word for “crown.” Under an electron microscope, the virus is studded with bulbous spike proteins that form a crown-like fringe around its surface. Those spikes aren’t just decorative. They act as grappling hooks that latch onto cells in your airways and pry them open for infection.
Beneath the spikes sits a fatty outer envelope, a waxy lipid layer that protects the virus’s genetic material while it travels between hosts. This is why soap and alcohol-based sanitizers work so well against coronaviruses: they dissolve that fatty shell, destroying the virus. Inside the envelope is a single strand of RNA, the genetic blueprint the virus uses to hijack your cells and produce copies of itself.
How the Virus Enters Your Cells
For SARS-CoV-2, and to varying degrees the other human coronaviruses, infection begins when spike proteins on the virus’s surface bind to a receptor called ACE2 on the outside of your cells. ACE2 receptors are found throughout the body but are especially concentrated in the lining of the nose, throat, and lungs, which explains why these viruses primarily cause respiratory illness.
Once the spike protein locks onto ACE2, it splits into two functional halves. One half holds the virus to the cell surface while the other drives the virus’s outer membrane into the cell membrane, fusing them together and injecting the viral RNA inside. From there, the cell’s own machinery starts reading the viral instructions and churning out new virus particles. Cells that lack ACE2 aren’t necessarily safe. Researchers have identified over a dozen alternative surface proteins the virus can use to get inside, which helps explain why SARS-CoV-2 can affect organs beyond the lungs, including the gut, blood vessels, and brain.
How Coronaviruses Spread
Human coronaviruses spread primarily through respiratory droplets released when an infected person coughs, sneezes, talks, or breathes. Larger droplets (those above 5 to 10 micrometers) tend to fall to the ground within about a meter, which is why close contact carries the highest risk. Smaller particles can linger in the air longer, especially in poorly ventilated indoor spaces.
You can also pick up the virus by touching a contaminated surface and then touching your mouth, nose, or eyes. The virus can survive on some surfaces for hours, though this “fomite” route is considered a less common path of infection than breathing in droplets directly.
Symptoms and Incubation
The four common strains produce what feels like any other cold. Symptoms typically include a runny nose, cough, sore throat, headache, mild fever, and general fatigue. In most healthy adults these resolve within a week or so. However, the same viruses can cause pneumonia or bronchitis in infants, older adults, and people with weakened immune systems or existing heart or lung disease.
COVID-19 symptoms can appear anywhere from 2 to 14 days after exposure, though most people develop them within about 5 days. The illness ranges enormously in severity. More than 80% of confirmed cases are mild, resembling a bad cold or flu. A smaller percentage develop difficulty breathing, chest pain, or confusion that requires hospital care. Loss of smell or taste became a hallmark symptom of early COVID-19 variants, though this has become less common with newer strains.
Long COVID
Some people infected with SARS-CoV-2 experience symptoms that persist for weeks or months after the initial illness clears. More than 200 symptoms have been associated with this condition, but the most commonly reported are fatigue that interferes with daily life, brain fog (difficulty thinking or concentrating), and post-exertional malaise, where physical or mental effort makes symptoms significantly worse.
Other frequently reported problems include shortness of breath, heart palpitations, headaches, sleep disturbances, dizziness on standing, joint or muscle pain, digestive issues, and changes in mood. There is no lab test that confirms long COVID. Diagnosis is based on a pattern of lingering symptoms following a COVID-19 infection. The condition can affect people who had mild initial infections as well as those who were severely ill.
Where Coronaviruses Come From
All known human coronaviruses are believed to have originated in animals before jumping to people. Bats harbor an enormous diversity of coronaviruses and are considered the primary reservoir for the group. The 2003 SARS virus likely passed from bats to civet cats before reaching humans. MERS-CoV is transmitted to people through contact with dromedary camels.
The precise animal origin of SARS-CoV-2 remains under investigation. Genetic evidence points to bats as the ultimate source, and early speculation focused on pangolins or other wildlife as a possible intermediate host. Environmental samples from a live-animal market in Wuhan detected SARS-CoV-2 genetic material in areas where animals susceptible to coronavirus infection had been kept, but those animals were no longer present when testing occurred, and no single species has been confirmed as the bridge between bats and humans.
Current Status
The WHO declared in May 2023 that COVID-19 no longer constituted a public health emergency of international concern, and by August 2023 it shifted from requiring daily case counts to requesting weekly reporting from member nations. Many countries have since stopped reporting COVID-19 data altogether or folded it into routine respiratory disease surveillance. SARS-CoV-2 continues to circulate globally, but at far lower levels than during the pandemic’s peak. The four common coronaviruses remain endemic, cycling through populations every year alongside influenza and other respiratory viruses.