How come that the virus, initially zoonotic, entered in human and transferred from human to human more efficiently than from animal to animal? Being a virus, for whom a human is a new host, SARS-2 is too generous in terms of mortality. Relatively tolerant immune answer in children to SARS-2, speaks about that cytokine storm may happen towards an intermediate loop in between the virus and immunity. My hypothesis is that virus uses normal human commensal flora to initiate its transfer from animal-to-animal and human to human. It also explains why antibiotics are efficient.