Re the discussion going on in the #youngwizards tag about this at the moment:
The word’s a back-formation deriving from Latin pullulare, “to sprout out, spring forth, spread, grow, increase”. Hence, something that spreads, grows, increases — and (borrowing from the OED’s entry for “pullulate”, “now usually with the connotation of rapid increase”. I pronounce it PULL-you-lus, though that’s a casual/Anglicized pronunciation, and properly it really ought to be PULL-oo-lus.
I decided the “origin noun” would be masculine because I didn’t want possible confusion arising from a form pullula that might have been mistaken for a Latin diminutive of the usual sort: and also because the “-us” ending sounded (to me) a little more unnervingly disease-y and/or organic. If that makes any sense.
…As it happens, BTW, the word (to my surprise) does occur in the classical literature, but only really rarely. See the entries in the Lewis & Short Latin lexicon at Tufts here (for the version that means a chick, pullus [chicken] + the masculine diminutive suffix -lus) and here (for the usage that derives from the adjective version of pullus that has to do with color).
A general note: where coined words are in play in the YW universe, always check the Latin and Greek end of things first. As languages of science, they take pride of place. I do coin from other languages, but not a lot and only in special circumstances. Also, watch out for medical terminology that creeps in through the back door (cf., for example, the “claudication” part of “temporospatial claudication”; it’s derived from an old word that can mean “to limp” but later came to mean the vascular narrowings / blockages that cause the limping).
Ladies and gentlemen, a how-to: author engagement with fandom. You may now applaud.