They don’t. In fact they try very hard to make their hosts just sick enough to spread them more efficiently. The deadly ones mostly screw up by being in the wrong host.
Covid is a perfect example of this. The exact same virus in a bat would have given them a runny nose, but it landed humans in the ICU or at the very least knocked us flat on our backs for a couple weeks. As the pandemic went on and the virus mutated, it became less and less deadly until it got to the point that if you catch Covid today it’s basically a really bad cold.
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