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The coronavirus refers to any virus belonging to a family of viruses called Coronaviridae or, in simple words, coronaviruses (CoV). There are about hundreds of these viruses which are mostly found circulating in the body of some specific animals such as bats, pigs, cats, and camels.
These viruses have obtained their name due to their characteristic appearance that resembles a crown, which in Latin is translated as “corona.” They have a typical shape with multiple spikes like projections on their surface, which resembles the points like projections on the crown.
Another typical feature is that these viruses have a single strand of RNA as their genetic material, unlike humans who have double-stranded DNA.
The coronaviruses are broadly divided into 4 types, namely alpha, beta, gamma, and delta. Amongst hundreds of Coronaviruses, only 7 have been found to cause infection in humans. For decades, amongst these 7 viruses, four are known to cause only mild to moderate illness. These viruses are quite common and are known to cause “common cold”:
1. H-CoV 229E (alpha coronavirus)
2. H-CoV NL63 (alpha coronavirus)
3. H-CoV OC43 (beta coronavirus)
4. H-CoV HKU1 (beta coronavirus)
Rest of the 3 viruses that infect humans are only identified in the past 20 years. These are known to cause more severe infection with significantly higher complications and death rates. These viruses are as follows:
1. SARS coronavirus (SARS-CoV): that caused severe acute respiratory syndrome (SARS) outbreak, which began in November 2002 and ended in 2004 (the beta coronavirus).
2. MERS coronavirus (MERS-CoV): that caused Middle East respiratory syndrome (MERS), which began in September 2012 and is still known to cause sporadic and localized infection outbreaks.
The human to human spread occurs mainly via respiratory droplets. It is believed that the tiny virus travels through the air, laden in the small respiratory droplets which are produced when a person coughs, sneezes, or talks. These respiratory droplets are very light and have the ability to float in the air and spread to nearby areas. Thus CDC has advised staying at least 6 feet or 2 meters away from another person.
Is coronavirus an airborne disease- know more?
There are many studies from different countries, which has shown that virus can spread from a healthy-looking infected person called as asymptomatic carrier. The person might not develop any symptoms at all or eventually develop symptoms in several days. One study has shown the infected person tends to shed the virus at its peak about 1 or 2 days before developing symptoms.
Spread from a surface: the virus can also spread when a person touches any surface with the virus on it. The virus gets access to the respiratory tract when the person touches his face, mouth, nose, or eyes. This mode of spread exists but is not considered as a primary method of transmission of the virus.
Spread from infected feces: transmission of the virus by consuming any item which is contaminated by infected person stool has been contemplated but not completely established. Some of the studies have found the virus in the stool of an infected person, which supports this notion.
To understand how SARS- CoV-2 virus infects humans, we need to understand a few things about the virus. The virus is mainly made of two components:
Outer protein layer: the outer covering of the virus is made up of protein, which has spike-like projections that attach the virus to the surface of the human cells and allows it to enter.
Inner genetic material: inside the virus, there is a single-stranded RNA in the form of the genetic material which can produce new viruses. However, the genetic material of the virus is in itself incapable of replicating and thus needs a host such as a human cell to produce more virus cells.
The SARS- CoV-2 virus is believed to have an affinity for the ACE2 receptors present on the human cells. These are primarily present in the respiratory system, especially the lungs. They are also found in others of the body, such as the heart, liver, kidneys, etc.
The virus is believed to get access to the human body by entering through nose, mouth, and eyes. It attaches itself to the mucous lining of the respiratory tract and enters the cells by interacting through ACE2 receptors. Once they are inside the human cells, they are known to hijack the protein-making machinery of the human cells. This allows them to use the human cells to produce millions and millions of copies of new viruses, which then spread to various parts of the body.
These viruses cause damage to an essential type of cells called ciliary cells. These cells have hair-like projections that beat rhythmically to move the mucus (protective fluid in the respiratory tract) out of the respiratory tract. By damaging these cells, the virus causes accumulation of the mucus within the lungs and thus restricts the absorption of oxygen from the air.
Along with this, the increase in the viral infection tends to stimulate the immune system, which causes infusion of the immune cells into the lungs to fight the infection. In moderate to severe cases of coronavirus, the infection tends to result in an exaggerated immune response, which often remains uncontrolled. It leads to damage to the normal cells of the lungs, which allows leaking of the fluid into the lung, further decreasing the capacity to absorb oxygen. As the damage increases, it eventually leads to respiratory failure, which needs to use for external support in the form of breathing machines or ventilators.
In a significant number of severely ill patients, the disease leads to damage to the other organs of the body. These organs then eventually stop functioning properly, leading to life-threatening complications.
It is believed that the damage is the result of marked uncontrolled immune reaction and/or the viral infection itself.
• Heart and blood vessels: it has been found to cause low blood pressure, irregular rhythm of the heart, and some form of heart damage in some of the severely sick patients.
• Kidney damage: several studies have shown the disease to cause acute kidney damage. One study in Wuhan found acute kidney failure in 27% of the patients. Most of these patients were either elderly or had some underlying chronic diseases such as hypertension or heart failure.
• Liver damage: A few of the studies have reported some damage to the liver cells.
• Immune system: the function of the immune system is to protect the body against invading microbes, including viruses. However, sometimes the severe infection can trigger an intense immune response. This, in turn, leads to damage to the body cells, causing multi-organ failure. A specific kind of exaggerated immune response called cytokine storm is also seen where an excessive amount of cytokine is released. This can lead to widespread damage to the parts of the body, including the brain.
• It has also been found that the children are relatively better protected and suffer from less severe infection. This is believed to be due to less complicated immune response seen in children against the infection.
• Brain and nervous system: multiple studies have shown severe Covid-19 cases developing neurological problems. The disease is now known to cause acute cerebrovascular disease, encephalopathy, agitation, and mental confusion.
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