Just as an aside on viruses. I have worked as a plant breeder all my life and in the last 10 years we as a company have become increasingly interested in viruses. My particular crop is barley. Here there is one gene (yd2) that confers true tolerance to one of the main viruses in barley, barley yellow dwarf virus, transmitted by aphids. If the plant has this gene, the viral load can be the same as in a susceptible plant but the plant does not exhibit any symptoms ( yellowing leaves, dwarfing and eventually death). This resistance was discovered in Ethiopian barley lines in the 1950’s and has been crossed into mainstream varieties.
There is also a new resistance gene, yd4, which has been crossed in from a closely related species, hordeum bulbosum. Here the virus cannot replicate in the plant and dies out.
In addition to these two known genes there are several others that confer a lower level of tolerance to this virus.
This sort of arms race will be going on all across the natural world In plants and animals. It is part of the natural order of things.