What we call an infection is the invasion of the body from the outside by minute living organisms, either plant or animal, and the establishing of them within the body, so that they grow and multiply. They carry on their metabolism just as do all other living cells and produce various chemical products as a result. There are many organisms living within our bodies whose metabolic products apparently do us no harm, and so we serve as hosts for these unbidden guests year in and year out without even knowing of their existence. The products from other kinds of organisms are poisonous to us, and when some of these organisms multiply within us we discover it, because we are poisoned and become ill. Only organisms whose products of metabolism are poisonous are counted ordinarily as causing infection. In the strict sense we might be said to be infected by the harmless organisms of which so many thrive within us, but in the usual use of the word we speak of one as having an infection only when his body has been invaded by organisms that produce poison. In the case of most kinds of injurious organisms their multiplication within the body unchecked would lead finally to its destruction. It is necessary, therefore, that the body have some means either of checking the development of the organisms or of neutralizing the poisons which they produce. The body has this power, and the machinery for it consists very largely of substances or structures in the body fluids. The detailed story of these is too complex to be told here. We shall content ourselves by saying that when an infection becomes established, as of scarlet fever, for example, the poisons that are produced are poured out by the organisms into the body fluids of the part where they happen to be located, and are taken up from there by the blood and distributed all over the body. The fever, headache, and other disagreeable symptoms are due to these poisons. The interesting thing about it is that the poisons themselves act toward the body as chemical regulators or hormones, exciting some or perhaps all of the cells of the body to a special kind of functional metabolism, which results in the manufacture of materials which neutralize the poisons. Thus, one of the very important properties of living protoplasm is to respond to the poisons from the metabolism of other cells by producing neutralizing material. Whether one dies from an infection or recovers from it depends on whether the cells are able to produce enough neutralizing material to prevent themselves from being killed or whether the poison is so abundant or so malignant that the cells are destroyed in spite of their activity in pouring out the neutralizing material.