Coronaviruses
are a group of viruses that cause diseases in mammals and birds. In humans,
coronaviruses cause respiratory tract infections that are typically mild, such
as some cases of the common cold (among other possible causes, predominantly
rhinoviruses), though rarer forms such as SARS, MERS, and COVID-19 can be
lethal. Symptoms vary in other species: in chickens, they cause an upper
respiratory tract disease, while in cows and pigs they cause diarrhoea. There
are yet to be vaccines or antiviral drugs to prevent and treat the virus.

Coronaviruses
were first discovered in the 1960s. The earliest ones discovered were
infectious bronchitis virus in chickens and two viruses from the nasal cavities
of human patients with the common cold that were subsequently named human
coronavirus 229E and human coronavirus OC43. Other members of this family have
since been identified, including SARS-CoV in 2003, HCoV NL63 in 2004, HKU1 in
2005, MERS-CoV in 2012, and SARS-CoV-2(formerly known as 2019-nCoV) in 2019.

The name
“coronavirus” is derived from Latin corona, meaning “crown”
or “wreath”, itself a borrowing from Greek κορώνη korṓnē,
“garland, wreath”. The name refers to the characteristic appearance
of virions (the infective form of the virus) by electron microscopy, which have
a fringe of large, bulbous surface projections creating an image reminiscent of
a crown or of a solar corona. This morphology is created by the viral spike (S)
peplomers, which are proteins on the surface of the virus that determines host
tropism.

After entry into
the host cell, the virus particle is uncoated, and its genome enters the cell
cytoplasm.
The coronavirus RNA genome has a 5′ methylated
cap and a 3′ polyadenylated tail, which allows the RNA to attach to the host
cell’s ribosome for translation.
Coronavirus genomes also encode a protein called
RNA-dependent RNA polymerase (RdRp), which allows the viral genome to be
transcribedinto new RNA copies using the host cell’s machinery.

The most recent
common ancestor(MRCA) of all coronaviruses has been placed at around 8000 BCE.
The MRCAs of the Alphacoronavirus line has been placed at about 2400 BCE, the
Betacoronavirus line at 3300 BCE, the Gammacoronavirus line at 2800 BCE, and
the Deltacoronavirus line at about 3000 BCE. It appears that bats and birds, as
warm-blooded flying vertebrates, are ideal hosts for the coronavirus gene
source.

Coronaviruses vary significantly in risk factor. Some can kill more than
30% of those infected (such as MERS-CoV), and some are relatively harmless,
such as the common cold. Coronaviruses cause colds with major symptoms, such as
fever and sore throat from swollen adenoids, primarily in the winter and early
spring seasons.