Fig. 1.

Origin of SARS-CoV2 infection or COVID-19 in humans. The BatCoV RaTG13 and RmYN02 are most closely related to SARS-CoV2. However, they are not able to cause infection in their natural form. So different theories have emerged, including the use of secondary animal hosts (which are not confirmed yet) by the virus to evolve there to cause infection to humans. However, another theory suggest the RaTG13 and RmYN02 evolved or undergone positive episodic selection in horseshoe bats. Fruit bats and ferret can also get the SARS-CoV2 infection through intranasal route. However, fruit bats do not develop visible symptoms and clear the infection, whereas ferrets can serve an animal model for asymptomatic SARS-CoV2 infection of the humans as they develop subclinical symptoms. The severe infection with SARS-CoV2 in humans causes severe pneumonia and the associated ALI/ARDS through generating the cytokine storm to cause death of the patient.