Some of the causes of disease, though, from their definite and invariable results, we may be perfectly aware of their presence, are yet of an occult nature, escaping all the tests devised to detect them. For instance, the miasm, as it is termed, which is the cause of intermittent fever, has never yet been detected in the atmosphere, by the application of any chemical test. And yet, no result in the wide range of disease is more definite and palpable than that which this miasm produces. And so secretly does it make its impression, that the disease sometimes lies dormant for a long period, even for weeks and months—the system all the while showing no signs of its presence. I once had a case of intermittent fever, which was not developed till a year had elapsed from the time of the patient’s exposure to the cause. The nature and mode of operation of the causes of many diseases are involved in mystery, and are subjects of discussion and dispute among medical men. The formidable, and often fatal malady, that results from a wound received in dissection, is attributed by some to a poison evolved in the decomposition of the body; while others suppose that it arises from the irritation of the wound simply, circumstances concurring to increase the irritation in one case, while it is left to subside in others. It is agreed, on all hands, that the contingencies on which the disease depends are not ascertained; and they are so often absent, that the cases in which the malady does actually occur bear a very small proportion to the whole number of instances in which such a wound is received. The same may be substantially said of the causes of typhus fever, cholera, scarlatina, &c. Some think that these diseases are caused by subtle poisons, which enter the system in various supposed ways; while others believe that they arise from causes which make _impressions_ merely upon the system, and thus awaken trains of morbid action. Whatever may be our opinion on these disputed points, the fact that there is so much secrecy in the operation of morbific influences, must, it is clear, make much of our knowledge of disease uncertain.