A coronavirus is a common type of virus that usually causes mild illnesses, but can lead to more serious infections
Coronaviruses are a group of viruses that usually cause mild illnesses, such as the common cold. However, certain types of coronavirus can infect the lower airway, causing serious illnesses like pneumonia or bronchitis. Most people get infected with coronaviruses at some point in their lives and the majority of these infections are harmless. The new coronavirus that causes the covid-19 illness is a notable exception.

Coronaviruses have extraordinarily large single-stranded RNA genomes – approximately 26,000 to 32,000 bases or RNA “letters” in length. Coronavirus particles are surrounded by a fatty outer layer called an envelope and usually appear spherical, as seen under an electron microscope, with a crown or “corona” of club-shaped spikes on their surface.

The virus that causes covid-19 is known as SARS-CoV-2. It appears to have first emerged in Wuhan, China, in late 2019. The outbreak has since spread across China to other countries around the world. By the end of January, the new coronavirus had been declared a public health emergency of international concern by the WHO.

The most commonly reported symptoms include a fever, dry cough and tiredness, and in mild cases people may get just a runny nose or a sore throat. In the most severe cases, people with the virus can develop difficulty breathing, and may ultimately experience organ failure. Some cases are fatal.

Coronaviruses replicate their RNA genomes using enzymes called RNA-dependent RNA polymerases, which are prone to errors, but genomic analysis so far suggests that covid-19 is mutating slowly, reducing the chance of it changing to become more deadly.

There are currently no vaccines or specific drug treatments for coronaviruses, but efforts to develop a vaccine are underway and HIV and Ebola drugs are being tested in people with covid-19.

On 18 March, the World Health Organization (WHO) said they had begun a trial of the most promising drugs, including the long-used antimalarial drugs chloroquine and hydroxychloroquine, a new antiviral drug called remdesivir and a combination of two HIV drugs called lopinavir and ritonavir. The HIV drugs will also be tested in combination with an antiviral called interferon beta.

On 22 March, several countries in Europe, including the UK, launched a collaborative trial of the same drugs, which will complement the WHO effort.

Many other potential treatments are being explored, particularly the possibility of giving people a dose of antibodies against covid-19, a strategy being explored with a clinical trial run by Johns Hopkins University.

Other severe coronaviruses
At least two other types of human coronavirus – Middle East respiratory syndrome coronavirus (MERS-CoV) and severe acute respiratory syndrome coronavirus (SARS-CoV) – are known to cause severe symptoms.

SARS-CoV first emerged in 2002 in Guangdong, China as an unusual pneumonia, which developed into life-threatening respiratory failure in certain cases. The virus rapidly spread across 29 countries, infecting more than 8000 people and killing about 800.

The MERS-CoV epidemic appeared in Saudi Arabia in 2012, with people experiencing similar symptoms to SARS-CoV but dying at a much higher rate of 34 per cent. Unlike SARS-CoV, which spread quickly and widely, MERS-CoV has been mainly limited to the Middle East.

Spread from animals to people
Coronaviruses are zoonotic, meaning that they can be transmitted to people from animals. Both SARS-CoV and MERS-CoV originally came from bats, though other animals – including camels in the case of MERS – can act as intermediaries that spread coronaviruses to humans.

Many of the early cases of covid-19 were traced back to a large seafood and animal market in Wuhan. The virus is thought to have come from bats, possibly via an intermediary animal. In response, Chinese officials enacted a ban on eating and trading wildlife in February, a measure that could become a permanent law.
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