SARS coronavirus 2 (SARS-CoV-2) is a positive-sense, single-stranded RNA coronavirus which causes the disease COVID-19. It used to be known as 2019 novel coronavirus (2019-nCoV) by the World Health Organization (WHO).
The virus started the 2019–20 coronavirus outbreak. The first suspected cases were reported to the WHO on December 31, 2019. Many early cases of this new coronavirus were linked to Huanan Seafood Wholesale Market, a large seafood and animal market in Wuhan, China. The virus may have come from infected animals. It is not certain that this place was the source of the pandemic.
Origins
The genetic material of this virus showed many similarities to SARS-CoV (79.5%) and bat coronaviruses (96%). This means the virus may have originally come from bats. Scientists did more experiments that showed the virus probably went from bats to an intermediate host, meaning another animal in between bats and humans. The viruses in that other animal changed over time until they could infect humans. Scientists are close to sure that the original animal was a bat but not sure what the intermediate animal was. Some scientists think it could have been a pangolin because there are coronaviruses that live in pangolins even though they are not exactly the same as SARS-CoV-2 or the ones in bats. Pangolins are an endangered species and buying, selling or moving them from place to place is illegal in China and many other countries. But their scales are an ingredient in many traditional Chinese medicines, so they are often sold on the black market.
Although SARS-CoV-2 began in bats, scientists noticed that the bats do not get sick and die from it. They think this is because the bats have very careful immune systems that fight the virus without causing inflammation. Scientists are studying bats because of this.
Scientists are still looking at SARS-CoV-2 infection in animals, but only a variant strain has been confirmed to spread to minks. It is also inducing fear in humans as there are cases being reported of transmission of COVID-19 from minks to humans. Cats, dogs and other mammals are said to at relative risk but how they can get infected is still a mystery.
Name
In early February 2020, the International Committee on the Taxonomy of Viruses' Coronavirus Study Group gave SARS-CoV-2 its official name, for sudden acute respiratory syndrome coronavirus number two. Before that, people called the virus "2019-nCoV," for "novel (new) coronavirus that appeared in 2019." The study group chose this name because the new virus was so similar to another virus that was already called SARS-CoV with no number.
How the virus causes disease
First, the virus enters the cell with the help of the S protein or spike protein expressed on the surface of the virus beyond its envelope. The spike protein attaches to the human cell receptor hACE2 or human angiotensin-converting enzyme 2 . ACE2 is found on the cells of the epithelial cell linings of the lungs, kidneys, and intestines. After the attachment of the S- protein to the virus, the S protein splits into its two components- S1 head and S2 stalk . The S1 head is now able to bind to the TMPRSS2 receptor which is a serine protease enzyme containing a transmembrane domain. The interaction of TMPRSS2 and S1 trigger the cell membrane of the human cell to merge with that of the virus leading to viral genome entry into cells via endosomes. The viral genome in the endosomes which convert to lysosomes is then processed by cathepsin, which is then replicated in the human cells and released after viral packaging and maturation in the cells to infect others. In October 2020, scientists said they had seen SARS-CoV-2 use another receptor, Neuropilin-1, to enter cells with the help of the furin cleavage site in its genome which was not found in any of its previous ancestors.
SARS-CoV-2 can infect many kinds of cells, but it mostly infects cells in the respiratory system, such as the cells of the nose, pharynx and lungs. Because it is in cells inside the nose, it can spread via aerosol or droplets when people breathe, talk, cough or sneeze.
The expanding part of the lungs, pulmonary alveoli, have two main types of cells. One cell, type I, absorbs oxygen from the air, i.e. gas exchange. The other, type II, produces surfactants, that help keep the lungs fluid, clean, infection free, etc. COVID-19 finds a way into a surfactant producing type II cell, and smothers it by reproducing COVID-19 virus within it. Each type II cell which is killed by the virus causes an extreme reaction in the lungs. Fluids, pus, and dead cell material flood the lung, causing the coronavirus pulmonary disease.