Bordetella pertussis is a Gram-negative, aerobic, pathogenic, encapsulated coccobacillus of the genus Bordetella, and the causative agent of pertussis or whooping cough. Unlike B. bronchiseptica, B. pertussis is not motile. Its virulence factors include pertussis toxin, filamentous hæmagglutinin, pertactin, fimbria, and tracheal cytotoxin. The bacterium is spread by airborne droplets; its incubation period is 9–10 days on average (range 6–20 days). Humans are the only known reservoir for B. pertussis. The complete B. pertussis genome of 4,086,186 base pairs was published in 2003.