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. 2020 Dec 2;99(1):93–106. doi: 10.1007/s00109-020-02012-8

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© The Author(s) 2020

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Fig. 1 — The emergence of a non-pre-adapted virus in a new host follows four consecutive phases: a Exposure (contact between the donor host and the recipient, which can be influenced by geographical, ecological, and behavioral factors). b Infection (the passage of the virus from the donor host to the recipient, which can be influenced by the ability of the virus to overcome the species barriers and by the compatibility with the new host: binding to the cell receptor, ability to complete the replication cycle, evasion the immune response, etc.). c Diffusion (transmission of the virus between subjects belonging to the new population, which can be influenced by the ability of the virus to complete its replication cycle in the new host and by the contact between the subjects that make up the new population). d Adaptation (evolution of the virus in the new host so as to remain in equilibrium within the population, which can be influenced by the genetic variability of the virus) [24]