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The cause of high COVID-19 virulence is because it is a pathogen that the human immune system has never encountered before.
What makes it strong is either because of its animal inhabitance or due to specific mutations in a member of the human Coronaviruses. According to recent evidence, bats are likely to be the source of initial contact (as the genome of Coronavirus that hosts them, resembles by 94% that of the COVID 19. Pangolins suspected to be the possible intermediate host. However, according to many researchers, the COVID-19 virus structure seems so complicated that it is highly unlikely to be natural, and it is possible that is laboratory-made and accidentally released.