Whenever a virus replicates, there is a chance for the new virus particles to develop mutations. If these mutations make them less contagious, they will quickly get outcompeted by the old lineage and die out. If the mutations make them more contagious, they will outcompete the other strains until they become the new dominant version that new mutations develop from.
The only upside is that viruses don't usually benefit from becoming more deadly; if anything, that makes them less able to multiply if they kill their hosts. (Exception: if the host becomes very contagious before dying to the virus, more lethal strains could still develop as long as they are able to successfully leap from person to person before their victims die.) So in the long run, many viruses tend to get more infectious but less lethal, since the mechanism that makes a virus lethal is usually complicated and most random changes to it from mutations tend to reduce severity, not increase it.
RyanW1019 t1_jdj280w wrote
Reply to comment by underbrownmaleroad in What happened to the old COVID variants, like Delta? Could they come back? by number1dork
Whenever a virus replicates, there is a chance for the new virus particles to develop mutations. If these mutations make them less contagious, they will quickly get outcompeted by the old lineage and die out. If the mutations make them more contagious, they will outcompete the other strains until they become the new dominant version that new mutations develop from.
The only upside is that viruses don't usually benefit from becoming more deadly; if anything, that makes them less able to multiply if they kill their hosts. (Exception: if the host becomes very contagious before dying to the virus, more lethal strains could still develop as long as they are able to successfully leap from person to person before their victims die.) So in the long run, many viruses tend to get more infectious but less lethal, since the mechanism that makes a virus lethal is usually complicated and most random changes to it from mutations tend to reduce severity, not increase it.