Bordetella pertussis, the etiologic agent of pertussis, is a fastidious gram-negative pleomorphic aerobic bacillus that attaches to ciliated epithelial cells of the nasopharynx, multiplies locally, and produces a wide array of toxins and biologically active products.

• B. parapertussis causes a similar, though typically milder, illness. B. holmesii causes up to 20% of pertussis-like syndromes.
• The most important toxin in B. pertussis is pertussis toxin, which has ADP ribosylating activity. The absence of this toxin in B. parapertussis may explain the milder illness.