We are not the only creatures who have to fight for our lives, every day of them, against an ever-present but unseen enemy. All other animals have to do the same, if they are big enough for any thing else to live inside them. There are parts of this earth where no cattle or horses can live, and where all the heavy work has to be by machinery or by hand, because there are flies there which bite all the larger animals, both wild and tame, and plant germs in their blood. After that the tamed creatures, at least, almost invariably die.

Animals, therefore, have their catching diseases just as we have. Sometimes they are the same as ours; more often they are different—things that dogs or cats or horses catch, and we do not. Once, I remember, it must have been more than sixty years ago, all the horses over entire states suddenly fell sick. One hardly saw a horse on the street or anywhere; and because that was long before the days of automobiles or trolley cars, people had to walk or to drive oxen, and sometimes even to harness up cows. But after a few months, the horses all either died or got well again. Those that got well had their white blood corpuscles trained to fight that particular germ; so there was no more serious trouble. But it would make you laugh to see a span of cows haul a milk wagon.

There is a distemper that kills puppies. There is a chicken cholera which wipes out whole flocks of hens. The monkeys in the menageries succumb to colds and consumption, which they catch from the people who come to look at them. When the plague enters a city, the cattle and the various small animals of the house begin to die first before the human beings, and thus often give them warning, and time to flee. In all cases, some particular living and growing thing breaks through into the blood, or gets a foothold somewhere in the body, and begins to poison it. Then the invaders and the blood cells fight it out—and the best man wins.

There is a disease called anthrax which attacks various animals, ourselves and the birds among the number. A hen, for example, fighting anthrax, will almost always win, if it is kept warm. The lymph cells fight better when the blood is hot. But if the hen’s feet are kept in cold water, then the anthrax wins. The cool blood gives it the advantage.

That is how we and all other creatures catch cold. Mere cold would never give us chills or grip or colds. But our being chilled, or getting our feet wet, or sitting in a draught, just cools down some part of the body enough so that the blood cells can’t fight quite so well. Perhaps it is only for a moment; but in that moment the enemy gets its footing. Some people say that the reason why a dog’s nose gets hot when he is sick, and why human beings and animals have fever when they have anything catching, is so that their bodies may be well warmed up, and their lymph cells may do their fighting in hot blood.

Nor is it only for human beings and animals that all these catching things lie in wait. Plants have their troubles as well. Perhaps you have noticed on wild cherry trees growing by the roadside, or on plumb trees, that sometimes almost every tree in a clump, and even every branch on a tree, will have a hard black lump, as large as your fist, growing out of the bark. The tree is sick; sick with a patching disease of trees called the black knot.

The disease itself is something like a mold. It breaks through at the growing tip of the branch where the bark is new and thin. Thence it grows down the branch sucking up the living substance of the plant and killing everything as it goes. The “black knots” are the fruiting of the invading plant, where it has broken through the bark and is growing fine dust-like “spores,” which are its seeds and will spread the trouble to neighboring trees. When a tree comes down with this trouble, the only thing to do is to cut off every diseased branch a foot or more below the lowest knot, and burn it up. Possibly then, we may get ahead of the enemy, and so save the rest of the tree.

Almost every cultivated plant has its diseases. The brown spots on leaves and fruit of a pear tree are the plant’s measles. The hard lumps in an apple are its mumps. There is a sort of black scarlet fever of wheat, rust it is called, which destroys a million dollars’ worth of grain each year. Some plants look wilted because they need water; some, because they are sick and need medicine.

And they get medicine, too. Sometimes the plant-doctor gives it by soaking the seeds in the remedy; sometimes by spraying it on the leaves with a pump and a hose. In any case, the medicine is some sort of poison that kills the living creature that makes the disease, but does no special harm to the cultivated plant.

We even speak now of diseases of soils. Because there are minute animals that get into soil and lessen its fertility so that no plant will do well in it. Green-house keepers often bake their earth before setting out plants in it, in order to cure it of any disease that it may have caught while the last crop was growing; while a healthy soil will catch a disease from a sick one, exactly as a healthy animal or plant will do.

The fact that most diseases of men, animals, plants, timber, and soils are living things is something that has not been known many years. Indeed, it is only since the twentieth century came in, that we have really made much of a start at finding out just what plant or animal is responsible for the sickness of any other. It is only a question of time, probably a few centuries at the outside, before we shall have killed off all the common diseases. Then nobody can possibly catch anything any more—measles, mumps, chicken pox, colds, grip, diphtheria, scarlet fever, or anything else. There will not be anything left to catch; and nobody will be sick any more, unless he eats more than is good for him, or does something else that he might just as well not have done.