Like Mike said, viruses and bacteria are very different beasts. The widespread use of antibiotics to kill bacteria means that if a mutation happens that's resistant to the antibiotic, that resistant bacteria has no competition because the non-resistant ones have been killed off. So the drug-resistant ones are free to proliferate wildly. We don't have the same situation with viruses, perhaps because we don't have widespread use of antivirals.
Mutations to viruses happen because the average infected person can have billions and billions of viral particles in their bodies. Just from a matter of sheer numbers, mutations are likely to happen. (This is another major reason for people to get vaccinated.) The article linked below gives way more info that any of us probably want (and most of it was over my head), but if you skip over all the different types of mutations, it gets to some basic numbers. Here's the take-away at the end:
"One billion viruses, per our one per 10,000 average, means three billion substitutions—and that’s just in one drop of mucus. Multiply that by the actual volume of active virus in a single body, then the number of active Covid-19 infections in a given populace, and before long you’ll find yourself well into the quintillions. In other words, the mutations that give way to viral variation are happening all the time at rates nearly inconceivable."
This is a great explanation of why trying to build herd-immunity by allowing people to just get Covid (and develop natural antibodies - if they survive it), is a really, really bad idea.
http://www.forbes.com/sites/williamhaseltine/2021/01/06/how-the-covid-19-virus-changes/?sh=20d3c668a084
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