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However, it is possible (and very rare) to randomly change the code in a way that makes the viruses more infectious and or more deadly (or gain function of some sort). This is rare because it's like taking code and blindly changing it for no rhyme or reason at any point and magically having coded it to do a specific function. There is a much much higher probability of just breaking the code. To gain function by randomly modifying code would be like winning the lottery many times in a row.
In the end, viruses don't do anything but enter cells, replicate, and randomly change. It is by random chance they become either weaker or stronger. After that it is by natural selection whether they continue or not. If the host dies before they can spread to the next host, or the immune system completely cleans it out first, that strain stops existing. Since vaccines only help the immune system remove certain strains, any strains it misses possibly becomes the next variant that is vaccine-resistant. In that way vaccines help filter out old strains, and allow new strains to exist.
TLDR; It's all randomness. Stop thinking viruses are assassins learning better ways to kill. They are just glitches in the matrix.