By "new" do you mean new strain or new species?

(e.g. SARS-CoV and SARS-CoV-2 are two strains of the same species; would a SARS-CoV-3 count as "new"?)

Also, what counts as "regularly"? (e.g. would you say that in 2016 measles "regularly" infected humans, even though its incidence was 20ish cases per million people per year? what about MERS, which is contagious among humans but usually with R0 < 1?)

@ArmandodiMatteo I'd say that regular could be changed to predictably, for more specificity. I will change the phrasing of "mutation" to "close mutation" by which I'd imagine that it must rely on a slightly different mechanism.

@ArmandodiMatteo However, I'm not an expert, so if you have any suggestions I'm happy to take them.