The genus Bordetella contains seven species. B. pertussis is by far the most important because it is the cause of classic pertussis (whooping cough). Nucleic acid homology and other analyses indicate that B. parapertussis and B. bronchiseptica are close enough to B. pertussis to be considered variants of the same species. B. parapertussis occasionallycauses a disease similar to, but milder than, pertussis. This is probably because it does not produce pertussis toxin even though it has a silent copy of the toxin gene. The remainder of this section will focus solely on B. pertussis.