In these experiments, the blood in six persons indicated the operation of the morbid cause, while each remained in a state of apparent health. Of these six persons, four were actually seized with yellow fever during the prevalence of the epidemic; and the other two, though they escaped any formal attack, did not escape indisposition. They were affected with headache, nausea, and other indications of disease, like hundreds besides, who were never absolutely confined to the house, and who never took any medicine, but who still experienced in nausea, giddiness, headache, pain in the extremities, and so on, abundant intimations of the presence of the poison. These examples may suffice to show how the exciting, may itself become a most powerful predisposing cause. The predisposition to subsequent attacks, after the system has once suffered from the disease, is very remarkable; that predisposition remains for a considerable period after convalescence and apparent recovery. Of this, striking examples continually occur both with regard to intermittent, and to continued fever. In fact, the disposition to relapse, remains until the constitution has recovered its previous strength and vigour, however distant that period may be. The influence of cold, moisture, fatigue, intemperance, constipation, anxiety, fear, and all the depressing passions, are likewise extremely powerful predisposing causes. They enable a less dose of the poison to produce fever, and they increase the intensity of the fever when it is established. They all act by weakening the resisting power inherent in the constitution, that is, by enfeebling the powers of life. CHAPTER IX. _Of the Treatment of Fever._ We have seen that the first indication of disease in fever is traceable to the nervous system; that the nature of this primary affection of the nervous system is unknown; that it may possibly be the commencement of inflammation, modified by the nature of the nervous substance, in which the inflammatory action has its seat, and by the nature of the cause that excites it, namely, a peculiar poison: or, on the other hand, it may possibly be something distinct from inflammation, but having a peculiar tendency to excite it. In either case, the inflammation that is present in fever, is peculiar and specific, differing essentially from ordinary or simple inflammation. Whether the affection of the nervous system consist merely of inflammation of the nervous substance excited by a peculiar poison; or whether it consist of some unknown condition of the nervous system to which inflammation is superadded, and by which the character of that inflammation is modified, the great practical result is the same, namely, that febrile inflammation and ordinary inflammation are not identical, and that the difference between the two affections is such as to require a very considerable modification in the treatment appropriate to each.