Then follows the period of return to health. The patient becomes a walking appetite, and, after weeks of liquid diet, will beg like a spoiled child for cookies or hard apples or pie, or something that he can set his teeth into. But his tissues are still swarming with the bacilli, and any indiscretion, either of diet, exposure, or exertion, at this time, may result in forming a secondary colony, or abscess, somewhere in the lungs, the liver, or the muscles. He must be kept quiet and warm, and abundantly, but judiciously, fed, for at least three weeks after the disappearance of the fever, if he wishes to avoid the thousand and one ambuscades set by the retreating enemy. Now, what has happened when recovery begins? One would suppose that either the bacilli had poisoned themselves, exhausted the supplies of nourishment in the body of the patient, so that the fever had "burnt itself out," as we used to say, or that the tissues had rallied from the attack and destroyed or thrown out the invaders. But, on the contrary, we find that our convalescent patient, even after he is up and walking about, is still full of the bacilli. To put it very crudely, what has really happened is that the body has succeeded in forming such antidotes against the poison of the bacilli that, although they may be present in enormous numbers, they can no longer produce any injurious effect. In other words, it has acquired immunity against this particular germ and its toxin. In fact, one of our newest and most reliable tests for the disease consists in a curious "clumping" or paralyzing power over cultures of the _Bacillus typhosus_, shown by a drop of the patient's blood, even as early as the seventh or eighth day of the illness. And, while it is an immensely difficult and complicated subject, we are justified in saying that this immunity is not merely a substance formed in the body, the stock of which will shortly become exhausted, but a faculty acquired by the body-cells, which they will retain, like other results of education, for years, and even for life. When once the body has learned the wrestling trick of throwing and vanquishing a particular germ or bacillus, it no longer has much to dread from that germ. This is why the same individual is seldom attacked the second time by scarlet fever, measles, typhoid, and smallpox. While, however, the individual may be entirely immune to the germs of a given disease, he may carry them in his body in enormous numbers, and infect others while escaping himself.