What exactly is a virus?

..."What exactly is a virus? Scientists don't include them among the living species of this earth, because they have no cells and can't reproduce or metabolise on their own.  All they are is a hollow shell that contains a blueprint for multiplication. Basically, they are dead. At least they are dead as long as they aren't docked onto an animal or a plant. Once they've made this connection they smuggle their blueprint into the host organism, forcing it to create millions of copies of the virus. In the process of multiplication, mistakes are always made, because viruses unlike cells have no built -in mechanisms to fix aberrations.
All kinds of mistakes, however, mean all kinds of new variations on the virus. It doesn't matter that many of them go nowhere, because there's always something useful in a rubbish heap. That's how viruses adapt quickly to new conditions and can attack their hosts more effectively. New mutations, especially, have the potential to kill. Normally, it doesn't make sense for a virus to kill its host because that would take away its opportunity to proliferate in the future. Only fresh mutations make mistakes like this, because they have't yet adapted sufficiently to be able to exploit their hosts without killing them.
Naturally, the opposite is true for the host: a host that has long relationship with a virus adapts over time so that the illness caused by the virus becomes relatively harmless instead of fatal. Chickenpox is an unfortunate example here. Europeans are well adapted to this illness, which usually strikes in childhood, but when the virus was brought to North America by white colonisers it devastated indigenous populations. In some areas chickenpox, in conjunction with measles and other diseases, killed up to 90 per cent of the population.
Animals face the same predicament. Globalisation creates similar conditions on continents that were new to them. Illnesses that native fauna don't know how to combat arrive in a new territory by imported animals and plants. African swine fever is one such disease. It's normally found in Africa, where bloodsucking soft ticks transfer the disease from one animal to another. In 2007 it was identified outside Africa in Russia, from where is spread to Europe, largely because people allowed this to happen. We don't know exactly how it was imported, but it probably arrived in a shipment of pork containing the virus. From there, the virus is likely to have spread through the illegal disposal of slaughterhouse waste and carcasses. The death rate for infected animals is extremely high: in fact, it's 100 per cent."...

(Peter Wohlleben, The Secret Network of Nature, 2017)