Seven coronaviruses are known to infect humans, but the broader coronavirus family is far larger. Scientists have formally classified dozens of species across four genera, and estimates suggest thousands more exist in wildlife, particularly in bats, that haven’t been identified yet.
The Seven Human Coronaviruses
Of the entire coronavirus family, only seven have made the jump to regularly infecting people. Four of them are common and relatively mild, while three cause severe disease.
The four common human coronaviruses, all identified since the 1960s, typically cause mild upper respiratory infections indistinguishable from a regular cold. Together, they account for 10 to 30 percent of upper respiratory tract infections in adults each year. They are:
- 229E, first described in the mid-1960s
- OC43, also identified in the 1960s
- NL63, identified in 2004
- HKU1, identified in 2005
The three severe human coronaviruses emerged more recently:
- SARS-CoV, which caused the 2002–2004 SARS epidemic. No cases have been reported since 2004.
- MERS-CoV, first reported in 2012, causing Middle East Respiratory Syndrome. Sporadic cases still occur.
- SARS-CoV-2, first reported in 2020, the virus behind COVID-19.
All three severe strains are betacoronaviruses, and all are believed to have originated in bats before reaching humans through intermediate animal hosts.
The Four Genera of Coronaviruses
Beyond the seven that infect humans, coronaviruses form a massive family divided into four main groups, or genera. Each tends to favor different types of hosts.
Alphacoronaviruses infect mammals, including dogs, cats, pigs, and humans. Two of the four common-cold coronaviruses (229E and NL63) belong to this group.
Betacoronaviruses also infect mammals and include the most medically significant viruses for humans. All three severe human coronaviruses fall here, along with viruses that infect cows, horses, mice, camels, and bats. OC43 and HKU1, two of the common-cold strains, are also betacoronaviruses.
Gammacoronaviruses primarily infect birds, especially poultry. Infectious bronchitis virus in chickens is the most well-known member.
Deltacoronaviruses infect both birds and some mammals, with pigs being a notable host. These are the most recently recognized genus and tend to cause disease in animals rather than people.
Coronaviruses as a whole have the largest genomes of any RNA viruses, ranging from 27,000 to 32,000 genetic letters. That unusually large genome gives them more tools for adapting to new hosts, which partly explains why they’ve spilled over from animals to humans multiple times.
Why Bats Are the Main Reservoir
Bats harbor an exceptionally wide diversity of coronaviruses and are considered the original evolutionary source of most, if not all, alphacoronavirus and betacoronavirus lineages. That includes the ancestors of every human coronavirus. Horseshoe bats in particular have been linked to the lineages behind both SARS-CoV and SARS-CoV-2.
The pattern is consistent: a coronavirus circulates harmlessly in bats for years, sometimes centuries, then passes through an intermediate animal (civet cats for SARS, camels for MERS) before adapting enough to infect humans. This process, called zoonotic spillover, is how all three severe human coronaviruses are thought to have emerged.
Thousands More Likely Exist
The formally named species represent only a fraction of what’s out there. A study published in Virus Evolution estimated the total number of coronaviruses circulating in bats alone at roughly 3,200, with a range of 1,200 to 6,000. The researchers arrived at this number by intensively sampling 27 bat species, finding an average of about 2.7 distinct coronaviruses per species, then extrapolating across the roughly 1,200 known bat species worldwide. Most of these viruses have never been sequenced or described.
That estimate covers only bats. Coronaviruses also circulate in rodents, birds, livestock, and other wild mammals, meaning the true global count is almost certainly higher. The vast majority of these viruses pose no known threat to humans, but the sheer diversity increases the statistical chances that another one could eventually adapt to infect people, as has happened at least seven times already.