coronavirus
, any
virus
belonging to the family Coronaviridae. Coronaviruses have
enveloped
virions
(virus particles) that measure approximately 120 nm (1 nm = 10
?9
metre) in diameter. Club-shaped
glycoprotein
spikes in the envelope give the viruses a crownlike, or coronal, appearance. The nucleocapsid, made up of a
protein
shell known as a capsid and containing the viral
nucleic acids
, is helical or tubular. The coronavirus genome consists of a single strand of positive-sense
RNA
(ribonucleic acid).
Coronaviridae is generally considered to contain two genera,
Coronavirus
and
Torovirus
, which differ in nucleocapsid
morphology
, the former being helical and the latter being tubular. Coronaviruses are important agents of gastrointestinal disease in humans, poultry, and bovines. In humans, a species known as
SARS
coronavirus (or
Severe acute respiratory syndrome coronavirus
) causes a highly
contagious
respiratory disease
that is characterized by symptoms of
fever
,
cough
, and muscle ache, often with progressive difficulty in breathing. The virus emerged in humans in 2002; it likely jumped to humans from an animal reservoir, believed to be horseshoe
bats
. The ability of SARS coronavirus to jump to humans undoubtedly required genetic changes in the virus. These changes are suspected to have occurred in the palm
civet
, since the SARS virus present in horseshoe bats is unable to infect humans directly (
see
SARS
).
More From Britannica
Why Is It Important to Know the Incubation Period of a Disease?
In 2012 another coronavirus capable of causing a severe
acute
respiratory illness later known as
Middle East respiratory syndrome
(MERS) was discovered in humans. The first case was found in
Saudi Arabia
, and others were reported within the following year in France, Germany, Jordan, Qatar,
Tunisia
, the
United Arab Emirates
, and the United Kingdom. All confirmed cases were directly or indirectly linked to the
Middle East
. Of all confirmed cases documented by 2019, roughly one-third had ended in death. The novel
MERS coronavirus
was similar to other coronaviruses known to have originated in
bats
and was thought to be passed from bats to other animals before being transmitted to humans.
Camels
were identified as one possible
reservoir
for the MERS virus.
In late 2019 a virus closely related to SARS coronavirus emerged in
Wuhan
, China. The
virus
, later named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), caused an illness known as
COVID-19
, which was similar to SARS and was characterized primarily by fever and respiratory symptoms. The virus was likewise highly contagious, spreading throughout regions of China, the
United States
, and Europe by early 2020, having been carried by travelers from affected regions. In March 2020 the
World Health Organization
declared the outbreak a
pandemic
, and travel to, from, and within many countries was severely restricted in an effort to control its spread. In many areas, schools and numerous businesses closed, and
stay-at-home guidelines were
implemented
, which strongly encouraged people not to leave their place of residence.
Vaccines
against SARS-CoV-2 were made available by the end of 2020, eventually allowing many businesses and schools to reopen.
Multiple variants of SARS-CoV-2 emerged during the course of the
pandemic
. In mid-2021, for example, the emergence of the so-called
Delta variant
, which was even more contagious than the original form of the virus, led to breakthrough cases among vaccinated individuals and was a significant cause of illness and hospitalization among the unvaccinated. Likewise, in late 2021, a variant named
Omicron
became the dominant strain of the virus in many places, including the United Kingdom and the United States. While the Omicron
variant
was significantly more infectious than the Delta strain, Omicron produced less severe illness in most persons. Similar to Delta, however, it was a major cause of hospitalization in unvaccinated individuals.