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may unite with S proteins that confirmed CoV infection and recognition. B⁰AT1 could be considered as important therapeutic target for SARS- CoV-2 infection suppression. The first identification of human coronaviruses (HCoV) occurred in the 1960s, as it was identified in the nostrils of patients with the common cold. There are two human coronaviruses, OC43 and 229E, which are responsible for a large proportion of common colds. Among humans, infection mostly occurs during winter and early spring. It is common for the instance of sickness to recur for the same person; one could become ill because of a coronavirus and then catch it again within 4 months later. This is possible because coronavirus antibodies do not last for a very long time. In addition, the antibodies for one strain of coronaviruses may be useless against other strains (1). All discovered coronaviruses that are causing illness for humans were originating from animals. Generally, these animals were either rodents or bats (32, 33). The spike proteins covering SARS-related coronaviruses contain many receptor-binding domains (RBD), which bind to angiotensin-converting enzyme-2 (ACE-2) receptor present in pneumocyte, cardiac cells, the gastrointestinal tract, and kidneys (34), which facilitates viral infection to the target cells. According to the phylogenetic analysis, the RBD of SARS-CoV-2 appears to be a mutated strain of its most closely related bat virus, RaTG13 (Rhinolophus affinis) (35). Due to this close relation, the SARS-CoV-2 was believed to infect people from bats after mutation. This mutation upregulated the RBD affinity to ACE-2 in humans and some animals like ferrets and Malayan pangolins (Manis javanica; an ant-eating mammal, which is illegally sold for traditional use in Chinese medicine), where it acts as intermediate hosts of SARS-CoV-2 (33), and decreased the affinity of RBD to ACE-2 found in civets and rodents. Little evidence indicated that SARS-CoV-2 originated from a manmade manipulation of an established coronavirus, but no supported evidence was present to support such hypothesis. In addition, Anderson et al. (35) indicate that the specific mutation, which was observed in the SARS-CoV-2 RBD, varies from what would have been expected based on genetic systems previously employed. However, scientists stated that “The other theories of (SARS-CoV-2) origin cannot currently be proved or disproved” (35). Bats were the source of both MERS-CoV and SARS-CoV that infect humans by civet cats and camels, respectively. Besides, bats were the natural hosts of SARS-CoV-2 as per the phylogenic studies of SARS-CoV-2 comparing with other CoVs, which showed that the novel virus is 96% identical to two SARS-like CoVs isolated from bats called bat-SL-CoVZX45 and bat-SL-CoVZX21 (2, 19, 29, 30). An intermediate host, which enables the novel virus to cross the species barrier to be able to infect humans, remains unknown. Ji et al. (36) suggested that snakes act as an intermediate host to the virus where homologous recombination within the S protein had occurred, transferring the virus from bats to humans. Another study in Guangzhou, China proposed that the ant-eating, long-snouted pangolins are the prospect intermediate host of SARS-CoV-2 depending on a 99% match in genetic identity between CoV discovered in pangolins and SARS-CoV-2 (37). A schematic-labeled diagram of coronavirus (SARS-CoV) and MERS-CoV and their transmission directly to humans from civet cats and dromedary camels, respectively, is illustrated in.Schematic-labeled diagram of coronavirus (SARS-CoV) and animal origins of human coronaviruses. Method of Transmission of Infection Transmission of human coronaviruses mainly occurs through air from an infected person to a healthy one by sneezing and coughing. Close physical contact seems to be transmitting the virus as well, such as rubbing or shaking hands and rubbing an object or surface contaminated with the virus and then touching one's lips, nose, or eyes before washing their hands. Transmission of theses virus rarely occurs through fecal contamination. Infection by human coronaviruses mainly occurs in fall and winter, bearing in mind that people can be infected by the virus at any time of the year and climate seems not to impede the transmission. Any person can be infected with one or more viruses of human coronaviruses during his life. The risk of being infected also extends to kids (1, 3, 20, 21). The amount of the replication or “R naught” (R0) is a mathematical term describing contagiousness (38). It is the number of people that might be infected from one diseased host. If the value of R0 is <1, the illness is considered to be not highly contagious. If the value of R0 is 1 or more, then the disease will spread among humans. The SARS-CoV-2 R0 value estimates ranged from 2.24 to as high as 3.58 (39). The mean R0 for seasonal influenza for comparative purposes is between 1.1 and 2.3 (regional and immunization variables), while for SARS, it ranged from 1 to 2.75. The marginally higher R0 for SARS-CoV-2 may be as so because it has a longer prodromal period, which increases the time of contagion of the infected host. Coronaviruses are usually believed to be most easily transmitted by respiratory droplets, which is to not be confused with airborne infection (40). Droplets are of a large size and appear to fall to the ground near the infected host and infect others only if a susceptible host intercepts the droplet before landing. The transmission of droplets is normally limited to short distances, <2 m. The airborne infection, however, requires smaller droplets, which may float with air currents and traverse longer distances. Under specific temperature and humidity conditions, these airborne droplets can remain afloat for hours. Airborne pathogens usually have higher R0 values, since infected particles can still be airborne after the infected patient left the premises, as occurred in airborne measles infection (R0 12–18) (41) and chicken pox (R0s 3.7–5.0) (42). When the infected droplets fall on surfaces, their capacity for viability on those surfaces is the driving factor of the possibility of them being transferred by touch. According to available studies of