Diphtheria. Another common and serious disease, until quite recently very fatal, is diphtheria. This is caused by getting into your mouth or nose the germs from another case of the disease. This disease also is most likely to occur in childhood, though it may attack a person of any age, and is always serious. It may be prevented from spreading by keeping children who have it shut up in rooms, or wards, by themselves and keeping all other children away from them, or from their nurses or those who have anything to do with them. Up to about thirty years ago, it was one of the deadliest and most terrible diseases that we had anything to do with. We knew absolutely nothing that would cure it, or even check its course; and nearly half of the children attacked by it died. About that time, however, two scientists, Klebs and Loeffler, discovered that, by taking some of the membrane, or tough growth that forms in the throat in this disease, and by rubbing it over a plate of gelatin jelly, they could grow on that gelatin a particular kind of germ. This germ, or bacillus, they then put into the throats of guinea pigs, and found that it would give them diphtheria. This is the way disease germs are discovered, or, as we say, identified; but of course this did not give at once any remedy for the disease. Scientists soon found, however, that, if a very small number of these bacilli were put into a guinea pig's throat, it would have diphtheria, but in a very mild form. If, when it had recovered, it was again infected, it would stand a much larger dose of the bacilli without harm. This made them suspect that some substance had been formed in the guinea-pig's blood that killed the bacillus or worked against its toxin, or poison; and soon, to their delight, they succeeded in finding this substance, which they called _antitoxin_ (meaning "against poison"). Then came the idea that if they could only get enough of this antitoxin, and inject it into the blood of a child who had diphtheria, it might cure the disease. A guinea pig is such a tiny animal that the amount of antitoxin which it could form would be far too small to cure a man, or even a child. So larger animals were taken; and it was finally found that the largest and strongest of our domestic animals, the horse, would, if the diphtheria germs were injected into its blood, make such large amounts of antitoxin that merely by drawing a quart or two of the blood--and closing up the vein again--enough antitoxin could be got to cure fifty or a hundred children of diphtheria. This treatment has not the slightest harmful effect upon the horse. The pain of injecting is only like sticking a pin through the skin, while the pain of bleeding is no greater than cutting your finger. There are now at our great manufacturing laboratories whole stables full of horses, for the production of this wonderful remedy. [Illustration: DEATH-RATE FROM DIPHTHERIA AND CROUP