Most of us will become infected with coronavirus at least once in our lifetime. This can be a worrying event for many, especially those who have only heard of one coronavirus, SARS-CoV-2, cause of the disease also known as COVID 19.
There are many more coronaviruses than SARS-CoV-2. It is actually a family of hundreds of viruses. Most of them are found in infected animals such as bats, chickens, camels and cats.
Occasionally, viruses that infect one species can mutate in such a way that it allows them to start infecting another species.
The first coronavirus was discovered in chickens in the 1930s. It took several decades for the first coronaviruses to appear that could infect humans in the 1960s. To date, there are seven coronaviruses that have the ability to cause disease in humans.
Four of them are endemic (found in specific people or in a specific area) and usually cause some mild illness, but all three can cause much more serious or even fatal illnesses.
Coronaviruses are found worldwide and are responsible for about 10 to 15% of colds, especially during the winter. The coronaviruses that cause mild to moderate disease in humans are called: 229E, OC43, NL63 and HKU1.
The first coronaviruses to be discovered to be able to infect humans are 229E and OC43. Both viruses usually cause a common cold and rarely cause a serious illness on their own. They are often detected at the same time as other respiratory infections.
In 2004, NL63 was first diagnosed in a baby suffering from bronchiolitis (infection of the lower respiratory tract) in the Netherlands. This virus has probably been around for hundreds of years, but we had not found it until then. A year later, in Hong Kong, another coronavirus was found, this time in an elderly patient with pneumonia. It was later named HKU1 and was found to be present in populations around the world.
But not all coronaviruses cause a mild illness. Sars (severe acute respiratory syndrome) caused by SARS-CoV was first identified in November 2002. The cause of this epidemic was not confirmed until 2003 when the virus genome was identified by the National Laboratory of Microbiology of Canada. Sars has many similarities to its current pandemic COVID-19. Older people were much more likely to suffer from a serious illness and symptoms include fever, cough, muscle aches and sore throat. But you were much more likely to die if you had Sars. From 2002 until the last reported case in 2014, 774 people lost their lives.
A decade later, in 2012, there was another outbreak involving a recently identified coronavirus: MERS-CoV. The first case of Middle East respiratory syndrome (Mers from Middle East respiratory syndrome) occurred in Saudi Arabia. There have been two more cases of Mers: in South Korea in 2015 and in Saudi Arabia in 2018. There are many cases of Mers each year, but the cases do not spread.
So why didn't Sars or Mers lead to pandemics? The R0 of both Sars and SARS-CoV-2 is approx two or three (although some more recent R0 estimates for SARS-CoV-2 it 's about five), which means that each infected person is likely to infect two or three other people. The symptoms of Sars were more severe, so it was much easier to identify and isolate patients.
R0 of Mers is less than one. It is not a very contagious disease. Most cases are associated with some contact with infected camels or a very close contact with an already infected person.
What is the R0 index? According to recent scientific articles, to determine how easily a virus is transmitted, experts have defined a "key number of reproduction", R0.
One of the major challenges in limiting the SARS-CoV-2 epidemic is that the symptoms may be very mild (some people may have no symptoms at all) but can infect many other people.
SARS-CoV-2 is not as deadly as Sars or Mers, but because it can spread undetected, the number of people it will infect and the numbers that will die will be much higher than any Koran virus we have ever encountered. .
This article was published in The Conversation by Lindsay Broadbent, Research Fellow, School of Medicine, Dentistry and Biomedical Sciences, Queen's University Belfast, Creative Commons.