<p><strong>Noun:</strong></p><ul><li>Any of various spiral-shaped motile bacteria of the phylum <em>Spirochaetes</em>, distinguished by twisting flagella that run lengthwise between the bacterium’s inner and outer membranes, and inclusive of the family <em>Spirochaetaceae</em>, noted for certain pathogenic species that cause Lyme disease, syphilis, and other illnesses.</li></ul>