**4. Evolution**

A virus is a tiny parasite, infectious substance of small size and basically structure that can multiply only in living cells of plants, animals or bacteria etc. The name is derived from a Latin word meaning "poison." Highly viruses have either RNA or DNA as their genetic material. The nucleic acid of viruses may be single or double stranded. The complete infectious virus element, called a virion, comprises of the nucleic acid and an outer shell is made up of protein [13]. Identifying evolving viruses are those that are freshly appeared or have recently expanded in predominance and/or geographical range uncovers several vital vague patterns. Primary virtually all emerging viruses have RNA more readily than DNA genomes. RNA viruses are more customarily familiar than DNA viruses. Additionally, almost all emergent viruses have an animal reservoir, such that the process of viral emergence can frequently be categorized as cross-species transmission. The extremely substantial exception for the rule is that cross-species transmission is central to viral emergence is HCV, which was first identified in 1989 but which is expected to have a considerably extended history in human populations [14]. In numerous cases, the specific cause of emergence, why the virus has traversed from animals and obsessed by humans can be assigned to ecological factors, involving to alterations in land use and deforestation. Although a multitude of such factors exist, either variations in the proximity of the donor and recipient populations, so that humans have an amplified coincidental of exposure to animal pathogens and variations in the size and density of the donor and recipient populations result in increase in both the exposure and the probability that continued networks of transmission will be established once a virus has inserted into a new species. Although biologists have assembled huge amount of knowledge how present day viruses evolve. When investigating the evolutionary history of most organisms, scientists can look at fossil record and homogenous historic evidence. While many of the viruses do not have the single ancestors. Generally recognizing the significance of ecology, it is also possible that genetic factors, whether in the host or further probably the virus, contribute to the process of disease emergence. As observed with other RNA viruses, SARS-CoV-2 is undergoing genetic mutations while adapting to new individual hosts. Several variants are reported, however only few are affecting public health. In United Kingdom, VOC 202012 (B.1.1.7 lineage) was described in December 2020 followed by B.1.351 lineage (501Y.V2) in South Africa. B.1.1.248/B.1.1.28/P1 (or 501Y.V3) was reported in Brazil in January 2021 and recently in California B.1.427/B.1.429 is discovered. As per WHO, 7 variants are of interest viz. B.1.525, B.1.526, B.1.427/B.1.429, B.1.1.28.2 alias P2, B.1.1.28.3, alias P.3 and B.1.616 [15].
