Figure 1.

Relationship of transmissibility, pathogenicity, and immunity with the anatomical proliferation of the virus, which is dependent on the receptor and coreceptor tropism.

An interesting working hypothesis is that the distribution of potential receptors and coreceptors on the human respiratory tract has been shaped by the deep-in-time evolutionary pressure of respiratory tract viruses in a way that a virus proliferating efficiently on the upper respiratory tract is unlikely to proliferate efficiently on the lower respiratory tract.