(plural spirilla), 1875, Modern Latin, diminutive of Latin spira (see spiral (adj.)). So called for their structure.
Entries linking to spirillum
spiral adj.
1550s, from French spiral (16c.), from Medieval Latin spiralis "winding around a fixed center, coiling" (mid-13c.), from Latin spira "a coil, fold, twist, spiral," from Greek speira "a winding, a coil, twist, wreath, anything wound or coiled," from PIE *sper-ya-, from base *sper- (2) "to turn, twist." Related: Spirally. Spiral galaxy first attested 1913.