re: there should be a word which describes relentless intermittent persistence ... like a viral infection which succumbs to a new vaccine but mutates and comes roaring back years later, as virulent as ever.

How about "chronic" or "recidivistic"?

chron·ic [adj.] American Heritage Dictionary:

Of long duration; continuing: chronic money problems.

Lasting for a long period of time or marked by frequent recurrence, as certain diseases: chronic colitis.

re·cid·i·vate [intr.v.] American Heritage Dictionary:

To return to a previous pattern of behavior, especially to return to criminal habits.

BTW there is a latin phrase which captures this sense of relentless returning: animus revertendi

It describes an animal, drawn from the wild, which "returns" relentlessly to the place where a human has provided a home [thru care or feeding].

In all other respects, the animal is the same as its brethern in the wild. It is distinguished only by this "animus" of returning, which allows the human to which it returns to claim ownership.