The conventional wisdom and experience tells us that viruses weaken over time, in terms of being deadly to their host. The strains that kill off their host are eventually crowded out by the ones that keep the host alive. But what would stop a variant from making humans (as a whole) perpetually ill? Is there the chance that, say 50% of us, will be coughing, aching and feverish at any given time in the future? Hopefully this depressing question doesn’t have a depressing answer.
Every year, adults have an average of 2–3 colds, and children have even more.
5-20% of Americas get the flu each year as well.
Mild contagious illnesses are already rampant in the population. Would it be likely that a full half the population would suffer at once? I’d guess not. If they did and gave it to the other half, what then? Do they just pass it back and forth every other week?
Colds and flu keep coming back because there are hundreds of variants, so your scenario of one variant taking over the population is doubtful.
It seems unlikely that, once infected and once the body’s defences are activated, that a virus could keep people continually ill. I’m not sure either viruses or the immune system work like that but I’m no expert.
Of course if it were the case then there would be an outward set of symptoms that someone could note and mitigate (e.g. masking, antivirals.social distancing etc.) and spread could be moderated in that way.
In that scenario a variant of that virus with fewer or milder symptoms that didn’t trigger such mitigations could potentially gain a competitive advantage.
Probably not 50%, because once 50% gets it, the other 50% will get it the following week, and with 100% seroprevalence we’ve easily reached herd immunity (for as long as it lasts). With omicron, many areas are hitting 25% PCR percent positive (possibly more, but test coverage is too thin to be certain). We’ll have broad seroprevalence by January, but that’s only going to last for a few months.
So it seems likely we’re headed for a future where, similar to the flu we’ll get a vaccine once a year (at least) for the latest variant. But COVID is going to be like a common cold except it kills tens of thousands of vulnerable people every year.