Before using it he had fourteen cases and three deaths; after, only three new cases and no more deaths. I would, however, hardly advise any human "coldie" to try such heroic treatment offhand, for the pungency and painfulness of formalin vapor is something ferocious, though the French physicians, with characteristic courage, are making extensive use of it for this purpose, with excellent results under careful supervision. Another curious straw pointing in the direction of the infectious nature of colds is the "annual cold," or "yearly sore throat," from which many of us suffer. When we have had it we usually feel fairly safe from colds for some months at least, often for a year. The only explanation that seems in the least to explain is that colds, like other infections, confer an immunity against another attack; only, unlike scarlet fever, measles, smallpox, etc., this immunity, instead of for life, is only for six months or a year. This immunity is due to the formation in the blood of protective substances known as _anti-bodies_, which destroy or render harmless the invading germs. Flabby, under-ventilated individuals, who are always "catching cold," have such weak resisting powers that they form hardly enough anti-bodies to terminate the first attack, without having enough left to protect them from another for more than a few weeks or months. Dr. Leonard Williams describes chronic cold-catchers as "people who wear flannel next their skins, ... who know they are in a draft because it makes them sneeze; who, in short, live thoroughly unwholesome, coddling lives." Strong and vigorous individuals may form enough to last them a year, or even two years. Now comes the question, "What are we going to do about it?" Obviously, we cannot "go gunning" for these countless billions of germs, of fifteen or twenty different species. Nor can we quarantine every one who has a cold. Fortunately, no such radical methods are necessary. All we have to do is to take nature's hint of the anti-bodies and improve upon it. Healthy cells can grow fat on a diet of such germs, and, if we keep ourselves vigorous, clean, and well ventilated, we can practically defy the "cold" devil and all his works. Here is the _leitmotif_ of the whole fascinating drama of infection and immunity. We can study only one phrasing here. We shall, of course, catch cold occasionally, but will throw it off quickly, and probably form anti-bodies enough to last us a year or more. How can this be done? First and foremost, by living and sleeping as much as possible in the open air. This helps in several different ways. First, by increasing the vigor and resisting power of our bodies; second, by helping to burn up, clean, and rid our tissues of waste products which are poisons if retained; third, by greatly reducing the risks of infection.