Then, it might be asked, of what consequence is it to insist on the disease being non-contagious? If fever-patients can give fever to others, it is a mere matter of words whether you choose to call it "contagious" or "infectious." It is, however, of the utmost consequence to fix the attention on the difference; because, if that is done, the real seat of the danger will be clearly seen, and those whose duty it is to enter the rooms of the sick will know that their danger rarely lies in touching the patient, and may be prevented by abundance of fresh air and scrupulous cleanliness. In order to emphasise this side of the truth my grandfather wrote as follows (and, though it may seem to require qualification, the _general_ truth of his remark will be admitted by all): "No fever produced by contamination of the air can be communicated to others in a pure air--there never was an instance of such communication." The form of poison given off from a fever patient is, besides, not so much to be feared as other forms of that poison, because, though it is potent, it has not a wide range; when let out into the fresh air, it is so far diluted that its power is reduced to a minimum. An epidemic, he asserts, can only arise from some cause sufficient to affect a whole district. Continually we are brought back to observe this universal cause of fevers; to see that, whether in the sudden falling off of an army to half its numbers, or in the prostration of a whole ship's crew on approaching shore, or in the plague devastating Cairo, this one source may be traced as the true one. Bad air comes from the marsh near which the army is stationed; bad air, poisoned by decaying vegetation, comes off shore to the ship; bad air enters the houses of Cairo. We are shown that Cairo is the birthplace of the plague, because it is a city crowded with a poor population; because it is built with close and narrow streets; because it is situated in the midst of a sandy plain at the foot of a mountain, which keeps off the wind, and is therefore exposed to stifling heat; and, above all, because it has a great canal which, though filled with water at the inundation of the Nile, becomes dry as the river gets lower, and thus emits an intolerable smell from the mud and from the offensive matter that is thrown into it. Besides being thus shown that, in all places in which epidemics appear, some sanitary defect may be found, we are shown that they come back and back to the same places, and that, if these defects are removed, the epidemics will not return. So we are led on to the great idea that they are _preventible_.