Eli5-If a virus isn’t technically alive, I would assume it doesn’t have instinct. Where does it get its instructions/drive to know to infect host cells and multiply?
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It is speculated that viruses long ago have been a part of cell machinery. E.g. every living cell has some systems that are responsible for making certain proteins, storing/reading genes, making copies of genes, etc.
One example of such a system is a plasmid – small circular DNA molecule that is able to travel from one bacteria to another. Their primary function is to provide cell with extra genes that may be useful for defense or for poison resistance. Also, some plasmids contain genes that specify how to make a plasmid itself.
Once plasmid has entered some foreign cell it is being read automatically and continuously by the cell machinery. And if it contains genes for self-replication, there will be new plasmids made, same as original.
Some plasmid could randomly mutate and incorporate some extra gene(s) which would help it replicate faster and degrade slower. These upgraded plasmids will soon out-compete old “inefficient” versions of themselves.
Another mutation could add some protective layer for the plasmid, which would help it to travel for longer distances between cells. Again, these extra-hardy plasmids will eventually out-compete more vulnerable ones.
After a few millions of years of such competition process you’ll get first viruses – pieces of DNA that tailored more to self-replication than to the cell survival.
There is no need for instincts – there are just all kinds of random mutations that happen all the time, and some of these mutations would help virus to infect & replicate, while others would hinder its spread. Those virus particles that got “good” mutations spread faster and infect more cells. That’s all to it.
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