Viruses  are infectious agents responsible for diseases that affect all animal and plant Species, and all the organs of these species. 

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

You should know that viruses are extremely simple organisms that are unable to make the molecules they are made of themselves. To reproduce, they therefore need to use the capacities of the cells of more evolved organisms than them. This forces them to enter these cells and divert the means of molecular synthesis for their benefit.

But, to achieve this, the virus must be accepted by the cells: in fact, the virus is for them a foreign body and they will seek to destroy it. This is what happens in general and this is what protects animals and plants from attack by viruses.

But, over millions of years of species evolution, some viruses have managed to be accepted by certain species which have no longer seen them as dangers. These species are called host species.

The relationship between a virus and a  host species  is very close, which explains why a viral disease generally affects only one species; another species, even a neighboring one, will resist the disease because it will continue to see the virus as a foreign body.

This is called the  species barrier  and that is why it is very rare for the same virus to infect different species.
Professor Jean-François COURREAU  ENVA (National Veterinary School of Alfort) in Maisons-Alfort .

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