City dirt has a great many different things in it. Among them, almost always, certain very small plants, far too small to be seen except with a pretty strong microscope. These are, in fact, a particular kind of bacteria. So we blow thru our skins, hole, dust, and bacteria. The hole heals over, but the bacteria stay inside.

Being living plants, they grow in the blood—like mold in bread or yeast in dough. Being living things, as they live, they make poisons. It happens that this particular plant makes an especially deadly poison, which goes straight for the nerves. Then the victim has convulsions, and almost always dies within a few days.

This is, in fact, the dreaded tetanus or “lockjaw,” which used to kill scores of boys and girls every Fourth of July. Sometimes, too, one catches it by stepping on a rusty nail, not because the rust on the nail does any special harm, but because a rusty nail is likely to be a dirty nail also, with the tiny living plants mixed in the dirt. We rarely get lockjaw from an ordinary cut with a sharp knife, because such a wound bleeds freely and washes itself out. The dangerous wounds are small deep holes and ragged tears, that give the little living plants a chance to hide and grow.

All catching diseases are like Fourth of July lockjaw. Measles, whooping cough, chicken pox, ordinary colds, grip, and many most dreadful sicknesses of which people die, all such are caused by some living thing which gets into our bodies, grows there, and living and growing, poisons us with its waste products. Some of these plants grow in the lungs, like that which causes consumption. In some, like diphtheria, the growth is in the throat. In summer complaint, which sickens the babies in the hot weather, the trouble is in the bowels. Even some sorts of baldness are due to growing things at the roots of the hair. Mostly, however, the plants grow in the blood. In any case, the poisons they make get into the blood; and there they poison the nerves, like the various alkaloids I told you about, or else they attack the blood itself, as the snake venoms do.

[The minute animal which causes the “sleeping sickness.”]

Some of these disease-making things, too, are not plants but animals. Such, for example, is the minute creature that causes malaria; and another that makes the dreadful “sleeping sickness” that every year is killing thousands of wretched negroes in Africa, in spite of all that can be done to prevent it.

They get into our bodies in all sorts of ways. Some come in the dust, when we breathe dirty air. Some come in dirty water. Some, a great many, come in dirty food, on lettuce and celery that have been carelessly washed, and especially in dirty milk. Some of the worst of all among them, the germs of typhoid fever, are carried on the feet of the common house fly, and planted all over the things that we are going to eat. Rats and mice also carry diseases—in their fur, on their feet, or even in their blood. So, too, do certain stinging, biting, and sucking insects; and when they bite or sting or suck the blood of some larger creature, they plant the seed of some disease in his body, where it grows and flourishes until the animal sickens and perhaps dies. No one, for example, ever catches yellow fever or malaria unless he has been stung by a mosquito which has already bitten somebody else with the disease. The mosquito picks up some of these living germs in the blood of one person, and sows them in the blood of the next; just as one might take seed from one field or garden plot and sow it in another.

All the catching diseases, then, from ordinary colds to pneumonia, and from measles and chicken pox to typhoid and scarlet fevers, are nothing in the world but living plants or animals growing in our bodies and poisoning us. We say that we catch the disease. Really the disease catches us. The disease is a living thing, that in very real sense, hunts for us, and catches us as a lion or a bear might do, or a poisonous snake. If we could kill these lions, bears, poisonous serpents, bacteria, and the rest, why then they wouldn’t get a chance to kill us. Then we should all live to old age—unless we poisoned ourselves, as I am sure some persons are quite foolish enough to do, or met with some accident that we could not help. But of course there are a few other diseases, like rheumatism and heart disease and indigestion, where the trouble may be with ourselves and not with any other living creature that gets after us.

Just to show you how one of these living, catching diseases manages to get on, and when one victim dies, changes over to another, I am going to tell you about something that I am sure you have already heard of either in your history, or else in stories that you have read about the Middle Ages, when the knights wore armor and the yeoman fought with spears and bows.