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 influenzæ 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.
The next fact in favor of the infectious character of a cold is that it begins with a chill, followed with a fever, runs a definite self-limited course, and, barring complications, gets well of itself in a certain time, just like the measles, scarlet fever, pneumonia, or any other frank infection.
Colds are also followed by inflammations, or toxic attacks in other organs of the body, lungs, stomach, bowels, heart, kidneys, nerves, etc., just like diphtheria, scarlet fever, or typhoid, only, of course, of milder form and less frequently.
Last, but not least practically convincing, colds may be traced from one victim to another, may "run through" households, schools, factories, may occur after attending church or theatre, may be checked by isolating the sufferers; and are now most effectually treated by the inhalation of non-poisonous germicidal or antiseptic vapors and sprays.
One of my first experiences with this last method occurred in a most unexpected field. An old friend, a most interesting and intelligent German, was the proprietor of a wild-animal depot, importing foreign animals and birds and selling them to the zoölogical gardens and circuses. I used often to drop in there to see if he had anything new, and he would come up to see me, to tell me his troubles and keep my dissecting-table supplied with interestingly diseased dead beasts and birds.
One day he came up in a state of great excitement, with a very dead and dilapidated parrot in his hand.
"Choost look, Dogdor; here's one of dose measley new pollies I god in from Zingapore. De rest iss coffin' an' sneezin' to plow dere peaks off, an' all de utter caitches iss kitchen him."