A CASE of measles or typhoid fever is not only a most unpleasant kind of practical problem, but a natural history phenomenon of a mysterious and interesting sort. Here is a person who wakes up apparently well and goes about his daily tasks as usual. Gradually he is conscious of some strange clog in the machine, a dragging of the wheels, such as we experience when a carriage passes from a good road into a sandy by-way. Pains and aches begin to be felt in head and back. The general weakness increases, and, with or without a sharp chill, the patient gives up and takes to bed. Fever has set in. The vigorous and active human animal of the morning has been changed in a few hours to a mere wreck of his former self. What has happened? What subtle force has produced so sudden and mysterious a catastrophe?The later history of such an attack is almost as remarkable as its inception. Most diseases go on and grow worse unless something definite is done to remove their exciting cause. If, however, your measles or typhoid patient be let alone, or only protected by hygienic precautions against certain secondary results, 99 times out of 100 in the case of measles, and 9 times out of 10 in the case of typhoid fever, he will get well. These are "self-limited" diseases, to use the old expressive term. They run a course of so many days or weeks, and then, unless death or some complication supervenes, there is a steady progressive recovery. The temperature falls, the mind clears, the strength returns, the patient is as he was before, with one important exception, that he is now, to a greater or less extent, and for a longer or shorter time, resistant or immune against the particular malady from which he has suffered. Think what a curious phenomenon this really is, divested of the cloak of familiarity with which it is commonly invested. What sort of strange process goes on in the body, which has a definite cycle like the life of an animal, fulfils its appointed round, and then draws to a close, leaving only the impress of immunity to mark its passage.