They don't have to, but there's nearly always a selective pressure to because of the coupling between immune escape and not causing horrible symptoms, which are themselves usually the immune system's response. Example: COVID doesn't kill the lungs through the virus itself screwing them up, but because it infected them rapidly and invokes an immune response there, creating inflammation and scar tissue from associated damage.
The reason it's a random walk is exactly what you're saying, though. A more severe variant can take over so long as it's better at spreading, which isn't solely a function of avoiding immune response, and hasn't exhausted its hosts. The last two variants that took over produce way higher viral loads and are still quite severe, for example.
A virus can also get less severe for a while, then get displaced by a more severe variant. Imagine, for example, if the next dominant variant were noticeably less severe and didn't cause permanent lung damage. Libs would declare victory (they're already trying to with omicron copium), but the next variant after that to take over could be more virulent simply because everyone is now more immune to the less severe one and not our huge backlog of shitty variants (let alone new ones).
And this stuff can take a very long time.
Just wanna stress that this is a real tendency in diseases and widely recognized in microbiology and people who study the evolution of infectious diseases but also it's a really bad excuse for thinking things will be fine on any acceptable timelinr. That excuse is also copium spread by a ruling class that is running the people through a COVID meat grinder and the people themselves who see that help is not coming and find this idea appealing.
Nothing says they have to. As long as a virus can infect new hosts quickly and efficiently there's no problem if it kills the one it's currently in.
They don't have to, but there's nearly always a selective pressure to because of the coupling between immune escape and not causing horrible symptoms, which are themselves usually the immune system's response. Example: COVID doesn't kill the lungs through the virus itself screwing them up, but because it infected them rapidly and invokes an immune response there, creating inflammation and scar tissue from associated damage.
The reason it's a random walk is exactly what you're saying, though. A more severe variant can take over so long as it's better at spreading, which isn't solely a function of avoiding immune response, and hasn't exhausted its hosts. The last two variants that took over produce way higher viral loads and are still quite severe, for example.
A virus can also get less severe for a while, then get displaced by a more severe variant. Imagine, for example, if the next dominant variant were noticeably less severe and didn't cause permanent lung damage. Libs would declare victory (they're already trying to with omicron copium), but the next variant after that to take over could be more virulent simply because everyone is now more immune to the less severe one and not our huge backlog of shitty variants (let alone new ones).
And this stuff can take a very long time.
Just wanna stress that this is a real tendency in diseases and widely recognized in microbiology and people who study the evolution of infectious diseases but also it's a really bad excuse for thinking things will be fine on any acceptable timelinr. That excuse is also copium spread by a ruling class that is running the people through a COVID meat grinder and the people themselves who see that help is not coming and find this idea appealing.