**2. Molecular characterization and pathogenic mechanism**

Coronaviruses are enveloped, spherical, or exhibit size and shape variation (pleomorphic) with a diameter in the range of 60–140 nm containing a positive sense, single-stranded RNA genome of approximately 26–32 kilobase size, the largest genome among RNA viruses [8, 18]. The genomic RNA contains multiple open reading frames (ORFs) for encoding 16 non-structural proteins (nsps) and four structural proteins like spike (S), envelope (E), membrane (M), and nucleocapsid (N). About two-thirds of genomic RNA is located in the first ORF (ORF1a/b) that helps in the translation of two polyproteins viz. pp1a and pp1ab at the 5′ end. Further, the subsequent proteolytic cleavages of polyproteins generate 16 nonstructural proteins (NSP). The remaining part of the virus genome encodes the four important structural proteins, E, S, N, and M, including other accessory proteins that interfere with the host's innate immune response [19, 20]. The invariant gene order is 5'-ORF1a-ORF1b-S-E-M-N-3′, with additional small ORFs in between the structural genes for encoding accessory proteins. The term 'corona' in Latin means 'crown' under the electron microscope observation; the spike protein projections of the virus appear as a crown, hence termed as 'coronavirus' (**Figure 3a**).

#### **3. Infection strategy**

2019-nCoV is extremely contagious and with very high transmission capacity, the virus is transmitted from person to person with ease. The transmission capacity is represented based on the reproduction number symbolized as R0 that signifies the average number of secondary cases (infectee) caused by the primary case (infector) in a population highly susceptible to infection [21]. The value R0 > 1 indicates the rapid spreading of the infection whereas R0 < 1 signifies the low extension capacity of the infectious disease. The R0 value of COVID-19 is in the range of 1.4–2.5, whereas severe acute respiratory syndrome coronavirus (SARS-CoV) is 0.67–1.23 and Middle East respiratory syndrome coronavirus (MERS-CoV) is 0.29–0.8; therefore, COVID-19 could be more easily transmitted [22, 23]. However, there are cases when an infected individual will not transmit the disease to anyone or can infect far more people than the standard transmission rate and the individuals are termed as "super-spreaders" [22]. In COVID-19, for the first time in early 2020, two patients were reported to be super-spreaders. One was a British national who had infected a dozen others whereas another suspect, a South Korean woman, had infected several dozens of others. The rate of initial spread is also dependent on the serial interval, which means the time gap between the onset of illness in an infector and in an infectee. The serial interval can be estimated by linking dates of onset of illness for infector-infectee pairs. The serial interval for COVID-19 is 4.4–7.5 days, whereas the mean value for SARS-CoV was 8.4 days, indicating the rapid transmission nature of COVID-19 [24].

It is reported in the literature that SARS-CoV-2 uses angiotensin converting enzyme 2 (ACE2) as its cell surface receptor and the binding of the S protein to the ACE2 receptor is the first step of viral infection followed by fusion with the cell membrane and subsequent viral entry to the respiratory mucosa [3, 18, 25]. As demonstrated in **Figure 3b**, after entry and un-coating, the translation of ORFs from the viral genomic RNA occurs for encoding non-structural proteins. Subsequently, the nsps assemble into the replicase-transcriptase complex (RTC) to facilitate RNA replication and transcription. First of all, full-length negative-sense anti-genome is synthesized using the genomic RNA as a template, and subsequently, the negativesense strand serves as a template for the synthesis of new genomic and sub-genomic *COVID-19: An Updated Insight of the Pandemic DOI: http://dx.doi.org/10.5772/intechopen.99097*

**Figure 3.** *(a) Basic structure of Covid-19, (b) life cycle in the host.*

RNA. Sub-genomic RNAs serve as mRNAs for the translation of structural proteins. The structural proteins and viral genomic RNA assemble in the endoplasmic reticulum (ER) - Golgi intermediate compartment (ERGIC) mediated by M protein to form the mature virions. Ultimately, the virions are transported to the cell surface through smooth-wall vesicles and released by exocytosis for subsequent rounds of infection [3, 25].
