**2. Structure of COVID-19**

Coronaviruses (CoVs) are affiliated with the Coronaviridae family, constituting a group of enveloped, positive-sense, single-stranded RNA viruses [10, 11]. They are named "CoVs" because of their crown-like structure under an electronic microscope [12–15]. Coronaviruses emerge from the Coronaviridae family, of the order of nidovirales. It was called the "coronavirus" due to the crown-like spikes on its periphery. Coronaviruses comprise a single-stranded RNA that is a tiny nucleic particle (65–125 nm in diameter) (**Figure 1**).

Four subcategories of coronaviruses—(a) alpha, (b) beta, (c) gamma, and (d) delta coronavirus—exist. These viruses were considered the agents of infections only in animals until an upsurge of SARS-CoV was identified in Guangdong, China,

**Figure 1.**

*Construction of respiratory syndrome that generates coronavirus in humans. [This file is licensed under the Creative Commons Attribution-Share Alike 4.0 International license; https://en.wikipedia.org/wiki/ COVID-19].*

in 2002 [14]. MERS-CoV, another pathogenic coronavirus, caused an endemic in the Middle East countries within ten years [15]. Around December 2019, Wuhan, a burgeoning business point of China, had experienced a flare-up of an unusual coronavirus, causing deaths of an estimated eighteen hundred people and infecting seventy thousand citizens in a fortnight of the outbreak. A formal notification announced that this virus was a member of the beta-type coronaviruses.

According to the ICTV, the main reason for COVID-19 is SARS-CoV-2; Chinese researchers have labeled this extraordinary virus as 2019-nCov or Wuhan coronavirus [16–18]. Statistically, 8098 individuals were infected by SRAS-CoV (2003), causing several deaths; the mortality rate reached 9% in 26 countries, whereas the novel coronavirus (2019) affected 120,000 individuals. In 109 nations, the infection caused a huge human loss, leaving a 2.9% mortality rate when this paper was published. Regarding communicability, SARS-CoV-2 is more intense than SRAS-CoV. The fundamental basis of the transmission was genetic rearrangements of the spike protein in the receptor-binding domain (RBD) of SARS-CoV-2, expected to have developed its transferability. In this review article, the source of human coronaviruses has been discussed as precisely as possible. The correlated infectivity and biological characteristics of MERS and SARS have been discussed with exceptional attention on COVID-19.
