To come nearer yet, did you ever catch cold when camping out? I have waked in the morning with the snow drifting across the back of my neck, been wet to the skin all day, and gone to bed in my wet clothes, and slept myself dry; and have lain out all day in a November gale, in a hollow scooped in the half-frozen ground of the duck-marsh, and felt never a hair the worse. Scores of similar experiences will rise up in the minds of every camper, hunter, or fisherman. You _may_ catch cold during the first day or two out, before you have got the foul city air, with its dust and bacteria, out of your lungs and throat, but even this rarely happens. How seldom one catches cold from swimming, no matter how cold the water; or from boating, or fishing,--even without the standard prophylactic; or from picnicking, or anything that is done during a day in the open air. So much for the negative side of the evidence, that colds are not often caught where infectious materials are absent. Now for the positive side. First of all, that typical cold of colds, influenza, or the grip, is now unanimously admitted by authorities to be a pure infection, due to a definite germ (the _bacillus influenzae_ of Pfeiffer) and one of the most contagious diseases known. Each of the great epidemics of it--1830-33, 1836-37, 1847-48, and, of most vivid and unblessed memory, 1889-90--can be traced in its stately march completely across the civilized world, beginning, as do nearly all our world-epidemics,--cholera, plague, influenza, etc.,--in China, and spreading, _via_ India or Turkestan, to Russia, Berlin, London, New York, Chicago. Moreover, its rate of progress is precisely that of the means of travel: camel-train, post-chaise, railway, as the case may be. The earlier epidemics took two years to spread from Eastern Russia to New York; the later ones, forty to sixty days. Soon it will beat Jules Verne or George Francis Train. So intensely "catching" is it, that letters written by sufferers have been known to infect the correspondents who received them in a distant town, and become the starting-point of a local epidemic. Of course, it may be urged that when we have proved the grip to be a definite infection, we have taken it out of the class of "colds" altogether, and that its bacterial origin proves nothing in regard to the rest. But a rather interesting state of affairs developed during the search for the true bacillus of influenza: this was that a dozen other bacilli and cocci were discovered, each of which seemed capable of causing all the symptoms of the _grip_, though in milder form. So that the view of the majority of pathologists now is that these "influenzoid," or "grip-like" attacks, under which come a majority of all _common colds_, are probably due to a number of different milder micro-organisms.