**3. Origin and prevalence of COVID-19**

It all started in the Hubei province's capital city, Wuhan, in December 2019, when several adults with severe pneumonia were admitted to the nearby hospitals. The surveillance team was triggered and collected the samples of respiratory patients for the etiologic study. It was investigated that numerous patients had contact with the Huanan wholesale seafood, where dead and live animals were sold and traded. At the end of December 2019, China declared the outbreak of this disease to the WHO. This virus had more than 95% homology with bat coronavirus (SARS-like bat CoVs) and more than 70% resemblance with SARS-CoV, and hence the virus was recognized as Coronavirus on January 7, 2020. The environmental samples obtained from the Huanan seafood market were also tested positive, which indicated that the virus took origin from the Huanan seafood market [15]. Though the Coronavirus originated from bats, the existing possibility of an intermediary animal that gets transferred to humans may be snakes or pangolins. Xu. Et al. have isolated SARS-CoV-2 from pangolin and found pangolin to be the potential intermediate host of the SARS-CoV-2 as it shows high similarity (99%) between the coronaviruses affecting the humans [16]. However, these current results are not sufficient to prove the potential host and intermediate of COVID-19. **Figure 1** shows a schematic view of crucial reservoirs and the mode of transmission of COVID-19.

According to Wu, JT, Leung et al. of York University, the estimated Basic Reproduction Number (R0), which means the average amount of secondary infection that patients may develop without intervention in a completely susceptible population, varies with several research groups [17]. Utilizing the Susceptible-Exposed-Infectious-Recovered (SEIR) model and Incidence Decay with Exponential Adjustment (IDEA) model, the estimated R0 value of novel COVID-19 was found to be 2.47–2.86 [18] and 2.0–3.3 [19] respectively, which is higher than other viruses of β–coronaviruses such as MERS-CoV (2.0–6.7) [20] and SARS-CoV (2.2–3.6) [21]. This elevated value of R0 points towards the fact that COVID-19 has a comparatively high transmission rate. It is also indicated from the overall case-fatality rate (CFR) that elderly male citizens are more prone to this Coronavirus, especially those with chronic health issues (heart disease, diabetes, hypertension) than other groups of the viruses. Thus, SARS-CoV-2 shows a high prevalence, and the population is easily susceptible to this virus. Among the RNA viruses, Coronavirus contains the most extensive genome sequence of about 26 to 32 Kilobases with 14 Open Reading Frames (ORFs). These ORFs code for 27 structural and non-structural proteins of the virus [22, 23]. Spike protein, membrane protein, envelope, and nucleocapsid, along with eight accessory proteins, lie in the 3′ end of the SARS-CoV-2 genome. A very high sequence resemblance is shared between structural proteins of SARS-CoV-2 and its predecessor human coronaviruses (hCoVs) (82%) (except in the 8a, 8b, and 3b accessory protein); suggests common molecular pathophysiology and pathogenesis among COVID-19, SARS, and MERS [24]. Genomic analysis has surmised the relevance of the Sans N gene in coronaviruses. Positive selection, mutation, and adaptation affect the pathogenicity and stability of the virus and might play an essential role in widespread infection in a large population [25]. This also poses a threat to the generation of newer strains of the virus that may result from mutation and adaptation, making the threat of transmission even more potent [26].

#### **Figure 1.** *Schematic view of the critical reservoirs and mode of transmission of coronaviruses.*
