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The lung is so irritated by every attempt to expel that which is causing the trouble it neither admits the air nor again easily expels it. The patient is seen to swell up and as if strangled holds his breath tightly in the middle of his throat . . . For they are without the troublesome coughing for the space of four or five hours at a time, then this paroxysm of coughing returns, now so severe that blood is expelled with force through the nose and through the mouth. Most frequently an upset stomach follows. . . . For we have seen so many coughing in such a manner, in whom after a vain attempt semiputrid matter in an incredible quantity was ejected.

Opinions differ as to why a clinically characteristic disease like pertussis was not described prior to de Baillou's description. Kloos and colleagues (1981) suggest that the absence of a clinical description of pertussis prior to the sixteenth century may reflect adaptation of a close genetic variant of B. pertussis to humans as recently as five centuries ago. Holmes (1940), in contrast, as noted by Mortimer (1988), attributed the lack of a prior description to an earlier preoccupation of physicians with other serious infections such as plague, smallpox, and typhus and to the possibility that they may have relegated the care of pertussis patients to ''old women."

The incubation period of unmodified pertussis averages 7 to 14 days, with a maximum of 21 days (Berkow, 1987). Clinically, pertussis can be divided into three sequential stages: the catarrhal, paroxysmal, and convalescent stages (Cherry et al., 1988; Mortimer, 1988). The onset of illness in the early catarrhal stage is subtle and is generally indistinguishable from that of a minor upper-respiratory infection. Early symptoms include rhinorrhea, mild conjunctival injection, sneezing, anorexia, listlessness, and a hacking nocturnal cough that gradually becomes diurnal as well. Fever is usually absent. During this time, coughing continues to increase in frequency and intensity and, by 7 to 10 days after the onset of illness, becomes explosive and episodic, heralding the onset of the paroxysmal stage. The disease is most infectious during the catarrhal stage, after which infectivity gradually declines.

The paroxysmal stage, which lasts 1 to 4 weeks, is dominated by severe episodes of coughing, which can occur 10 times or more in a 24-hour period. Each paroxysm is characterized by five or more rapid short coughs followed by a deep hurried inspiration. It is this hurried inspiration through a narrowed airway that produces the characteristic whoop.

Paroxysms are thought to be caused by efforts to expel the thick mucus that characteristically accumulates in the tracheobronchial tree. During such episodes, copious amounts of this mucus are expelled, often causing vomiting and, in infants, choking spells and cyanosis. The child is often exhausted following a paroxysm, although he or she can appear happy and relatively normal between episodes. Multiple paroxysms tend to occur within