That's the simplified theory, sure. But in practice natural selection does not guarantee the removal of (ostensibly) disadvantageous characteristics.The laws of nature are that every living thing wants to propagate itself. You can argue if viruses are techinically 'alive' since it needs a host to reproduce but it contains rna so it counts. A virus that kills it's host quickly isn't very advantageuous so natural selection will always favor less deadly variants of the same virus. It's how they speculate the common cold most likely also originated as a similar coronavirus as covid. Eventually covid will become more infectious but relatively harmless. Ofcourse the question is how long that takes. Probably like half a century. Like with everything nature takes it's time lmao.
Imagine, for instance, a mutation that increases the internal replication of a virus by many orders of magnitude. The host dies more quickly. But the host also reaches the infectious stage more quickly, and is more strongly infectious whilst at that stage. The lower lifespan of the host has been offset; natural selection can favour that mutation.
On a side-note, the reason that there's debate over whether viruses are living things or not has nothing to do with the fact they require a host organism. After all, plenty of parasites require host organisms-- liver flukes, parasitic fungi, etc-- and are categorically life forms. Viruses, on the other hand, have no metabolism, and have no cells (usually considered an essential component for a living organism). They act solely as errant agents within other living things.
They are subject to natural selection, of course. But natural selection is often misunderstood as some kind of "will" or "goal" existing within the organism to survive. That's not what it is. Lower life-forms such as bacteria or quasi-life-forms like viruses do not have "wills" and "wants", and may well act in ways that are not advantageous to themselves from a macrocosmic perspective, but have just simply not been selected out for various reasons.