The name of this grammar structure?

Question

> It is a serious, ***and sometimes fatal***, disease that may become epidemic in crowded, unsanitary living conditions.I'm trying to find the name of grammar structure (a very technological term, not just something like "parenthetical phrase to add additional information", as I seek to find every information about it) of "and sometimes fatal", a parenthetical phrase and what seems to be a coordinative adjective.I just do not understand how it is grammatical to wrap that conjunction and adjective in commas like that. Isn't comma supposed to come before the conjunction only when a new independent clause is beginning?

Answer 1

I would have said, “It’s a serious and sometimes fatal disease . . . “ There are two adjectives, “serious” and “fatal” both describing the noun “disease.” I would say that setting them off with commas to make a parenthetical would be incorrect.