Viruses are infectious agents that cause diseases in all animal and plant species and in all the organs of these species. 

However, in the vast majority of cases, a disease will affect only one species and, often, a single organ or even a single tissue.

Viruses are very simple organisms incapable of making by themselves the molecules of which they consist. To reproduce, they therefore need to use the cells of more evolved organisms. This requires that they penetrate these cells and exploit to their advantage their means of molecular synthesis. 

But, to achieve this, the virus must be accepted by the cells. The virus is in effect a foreign body and the cells will therefore attempt to destroy it. This is actually what happens in most cases — the cell protects animals and plants from viral attacks. 

However, over the course of millions of years of evolution, certain viruses have succeeded in getting certain species to no longer see them as a danger. Such species are called host-species. 

The relationship between a virus and a host-species is very close, which explains why a disease affects in general a single species. Other – even very closely related – species will develop a resistance to the disease because it will continue to view the virus as a foreign body.

This is what we call the species barrier and it is why it is very rare that a single virus can infect different species.


Professor Jean-François COURREAU 
ENVA (Ecole Nationale Vétérinaire d'Alfort) at Maisons-Alfort.