Know how to use "fewer" and "less"? Find out.
c.1300, "noxious animals," from Anglo-French and Old French vermin, from Vulgar Latin *verminum "vermin," possibly including bothersome insects, collective noun formed from Latin vermis "worm" (see worm). Extended to "low, obnoxious people" by 1560s.
vermin ver·min (vûr'mĭn)
n. pl. vermin
Any of various small animals or insects, such as rats or cockroaches, that are destructive, annoying, or injurious to health.