Severe Acute Respiratory Syndrome Coronavirus 2
The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) (in English: Severe acute respiratory syndrome coronavirus 2), initially called "2019-nCoV" (in English: 2019 novel coronavirus), sometimes called "coronavirus" Wuhan virus", or "COVID-19 virus", is a positive-stranded (linear genome) RNA virus. It is contagious among humans and is the cause of the COVID-19 disease, of which there is an ongoing pandemic. The World Health Organization (WHO) considers that bats are the most likely natural reservoir of the virus, although some differences between the viruses found in bats and those found in humans suggest that humans were infected through an intermediate host. The first known infections were discovered in Wuhan City (Hubei Province, China) in December 2019. Because the discovery was in Wuhan, it is sometimes called "Wuhan virus" or "Wuhan coronavirus", although the World Organization (WHO) advise against using location-based names. To avoid confusion with severe acute respiratory syndrome (SARS) disease, WHO in public releases sometimes refers to the virus as the "virus responsible for COVID-19" or "COVID-19 virus". Both the virus and the disease are often referred to as "coronavirus" or "new coronavirus" by the general public, although scientists use more precise terms.
Current evidence indicates an origin through a zoonotic leap. The strain was first described in Wuhan, but it is still unclear what the exact geographic location of viral transmission to humans was, nor when the strain became pathogenic (began to produce infectious diseases in the hosts). The COVID-19 virus is genetically close to the coronaviruses found in bats, from which it probably originated. A nucleic acid sequence in bats of the species Rhinolophus affinis collected in Yunnan province, also in China, revealed a 96% similarity to SARS-CoV-2. But it is thought that before it was introduced to humans, an intermediate animal reservoir such as the pangolin was also involved. From a taxonomic point of view, this virus is classified as a strain of the coronavirus species related to severe acute respiratory syndrome (SARS-CoV), but its receptor binding domain (RBD) of the spike protein is different and, therefore, its binding with the human cell ACE2 receptor would not be effective. On February 7, 2020, investigators in Guangzhou City (Guangdong Province, China) discovered a pangolin sample with a nucleic acid sequence 99% identical to SARS-CoV-2 , with the difference of just one amino acid. Although Chinese law protects these mammals, it is still common for them to be traded illegally for use in traditional Chinese medicine. Also in the state of Texas (United States), microbiologists and geneticists found evidence of rearrangement in coronaviruses, which suggests the involvement of pangolins in the origin of SARS-CoV-2. However, coronaviruses in pangolins found to date share only 92% of the genome with SARS-CoV-2, which is insufficient to prove that pangolins are the intermediate host. In comparison, the SARS virus responsible for the 2002-2004 outbreak shared 99.8% of its genome with the civet coronaviruses. As well as the origin, there is also no consensus for the natural selection of the virus: it is not known if it was performed before or with zoonotic overflow, in the intermediate host or in humans, respectively.
Human-to-human transmission of SARS-CoV-2 was first confirmed during the 2019-20 coronavirus pandemic. The main form of transmission is droplets produced in the system.