Old English flea "flea," from Proto-Germanic *flauhaz (source also of Old Norse flo, Middle Dutch vlo, German Floh), perhaps related to Old English fleon "to flee," with a notion of "the jumping parasite," but more likely from PIE *plou- "flea" (source also of Latin pulex, Greek psylla; see puce).
Chaucer's plural is fleen. Flea-bag "bed" is from 1839; flea-circus is from 1886; flea-collar is from 1953. Flea-pit (1937) is an old colloquial name for a movie-house, or, as OED puts it, "an allegedly verminous place of public assembly."
"A man named 'Mueller' put on the first trained-flea circus in America at the old Stone and Austin museum in Boston nearly forty years ago. Another German named 'Auvershleg' had the first traveling flea circus in this country thirty years ago. In addition to fairs and museums, I get as high as $25 for a private exhibition." ["Professor" William Heckler, quoted in "Popular Mechanics," February 1928. Printed at the top of his programs were "Every action is visible to the naked eye" and "No danger of desertion."]