Bordetella pertussis is the causative agent of pertussis, also called whooping cough or the cough of 100 days. Infection can result in significant morbidity and mortality, particularly in young infants. Prior to the availability of effective vaccines, pertussis was a major cause of childhood disease. With the advent of such vaccines, the incidence of disease declined dramatically into the 1970s. However, pertussis is still present, with peaks occurring every 3 to 5 years, and the number of cases has been increasing in the United States since the 1980s. With recent reports of numerous outbreaks of pertussis, there is heightened interest in the control and diagnosis of the disease. Efforts to increase immunity through vaccination and also to improve the clinical and laboratory diagnosis of the disease are very important. Part I of this two-part article will review a recent outbreak of pertussis that occurred in California and discuss the biology of the genus Bordetella, followed by the clinical presentation of disease and recommendations for recent vaccination protocols and guidelines for diagnosis. Part II of this article will be published in the August 15 issue of this newsletter and will review laboratory methods available for diagnosis, along with their problems and pitfalls.