The hardest battle we have to fight is with these living diseases. They kill more people in each year than have perished at the hands of the enemy in all the wars we have ever fought. During our war with Spain, the flies alone killed in camp four times as many of our soldiers as the Spaniards killed in battle. Every day of our lives in war and in peace, we are fighting for our lives against these unseen foes of pestilence and disease.

This is how we carry on the campaign. Our first line of defense is keeping clean. Every city now-a-days has men who watch its water supply. The surroundings of its ponds and reservoirs are carefully guarded. If necessary, the water is filtered before it goes into the street pipes. Always, if the city is half civilized, it filters all its sewage before turning it into the rivers. Thus, if there are any living animals or plants in the water, they get strained out. In many houses, where the people are especially careful, they strain or filter the water once more, or boil it till they kill every living creature therein.

Then every city has other men to look after its milk. Because milk, being good food for us, is good food also for other living creatures; so that if one single germ gets into a bottle, it will shortly grow to many millions, and play sad havoc with the family that uses it. Careful people, too, do not depend on the city to keep their milk clean, but see for themselves where it comes from; especially if there are children, for children not only drink much milk, but are peculiarly liable to catch the diseases which come in it.

Careful people, in addition, look out that all their food is clean. They see that none has been kept out where dust may fall in it, where rats or mice may brush against it, or where flies may track over it. In all these ways, the seeds of diseases may get sown in our food. One ought to make sure that there are no rats, mice, or flies in the house at all; and one ought to make sure also that all raw foods, like lettuce and celery are thoroly washed, for these often carry the eggs of certain animals, living eggs which will hatch out inside anybody who swallows them, and not be at all to his advantage.

That, then, is our first line of defense—keeping our food clean, lest the enemy enter through our mouths. Our second defense is to keep the air we breathe, clean and fresh and dust free, lest the enemy attack us by way of our lungs, and we die of pneumonia or consumption, or sicken of common colds and the grip. In all this, we are like a country with a powerful navy which can prevent the enemy from making a landing on its coast. So long as we keep these minute foes out of our houses, and still more so long as we keep them out of our cities, they cannot get near enough to us to do us any harm.

But when the enemy has broken through our first line of defense, and begun to lay siege to our bodies, we still have a second line of fortifications to protect us. One thing our skins are for is to stop germs. So long as we keep a whole skin, it is pretty hard for many sorts of germs to gain an entrance. And where the skin is thin, as it is inside the mouth and nose, there is always a somewhat thick and sticky mucous to catch the germs; while the eye has tears to wash them out. We are like fortified cities; and we must not let the enemy make a breach in the wall.

So we must be careful not to neglect even small cuts, sores, scratches, and the like, thru which the foe can enter. People sometimes get most loathsome and dreadful diseases by drinking from dirty cups when the skin of their lips is cracked in cold weather; while the little plant that causes blood poisoning, often gets in by way of a cut so small as to be hardly noticed. No child would ever die of lockjaw, if he did not first cut or blow a hole in his skin.

Strangely enough, a great many of the lesser catching diseases of children, measles and colds and the like, make their way into the blood because of holes in the teeth. A hollow in a tooth gets filled with food. A germ or two finds its way into this food, and grows there till it becomes many millions. So naturally, an assault by an army is more likely to be successful than an assault by a mere handful. Merely by keeping the teeth and mouth clean, and by having a dentist stop the holes in the teeth, one can cut down to less than half, the number of days’ sickness in a year. Some people go beyond this, and always after they have been in dust or bad air, wash out their noses with some fluid that will dispose of any of the enemy who have lodged there. In fact, people who have suffered almost continually from colds and grin are sometimes cured at once and completely, by merely a slight operation on the nose, which opens up to the fresh air some hollow in which the besieging hosts were wont to lurk.

Our second line of defense, then, is keeping skin and teeth and nose clean and whole, that the enemy may have no place wherein to hide, and no breach through which to enter. If this gives way, then we must engage the enemy hand to hand.

Then it gets to be a pretty even thing between us. The invaders try to poison us, and we try to poison them. We form an acid in our stomachs—the same acid, by the way, that tin-men use to solder with; one can buy it anywhere. Regularly, we use this to digest meat with; but if any living things get into our stomachs, we give them a dose of this acid, and usually bowl them over.

If they get by this, we poison them with bile. The bile is made in that most useful organ, the liver, and is partly waste matter, more or less hurtful to the body, which we are getting rid of. So since we don’t want it to poison us, we use it to poison other living creatures inside us.

That, then, is our third line of defense. If the enemy gets by that, then the war has to be fought out in the blood. Even if we win, we shall suffer damage, and very likely take to our beds and have to call on the doctor for reinforcements.

The blood itself, good healthy blood, that is, will poison the germs; for the blood of any sort of creature, as I have explained, will poison any other sort. Besides that, we manufacture special poisons with which to combat each special creature that makes a special poison to assault us. So each side poisons the other, and the battle becomes a question of which can kill the other first. In one case, we are sick and recover; in the other case, we are sick and die.

When the doctor takes a hand in the battle, he begins by giving us food and medicine, to make us strong for the war. Sometimes, besides these, he gives us something which, while it harms us a little, harms the enemy a great deal more. In a few cases now-a-days, he pumps into our bodies some of the blood of another animal, generally a horse, which has been fighting the same disease, and so has its hand in and can manufacture many times as much stuff to kill the germs as we can. Many, many people have been saved by this means, who otherwise would certainly have died.

In a few cases, too, small-pox especially, by means of vaccination, long before the enemy attacks us, we can be loaded up with ammunition to repel the invader when it does come. The vaccination gives a mild form of the disease (not so very mild perhaps you think, when your vaccinated arm gets nice and red); we defeat this triumphantly, but we manufacture so much “anti-toxin” to do it with, that for years afterwards we are all ready for the first germ that shows its face in our blood, and slay him before he gets a chance at us. The “anti-toxin,” as you know, is the substance which we make with which to get back at the germs that are trying to poison us with the “toxin” which they make.

But even the battle in the blood is not fought quite in the open. We have, so to say, breastworks and rifle pits, where we can still make a stand, even after the invaders have gained a footing in our bodies.

Perhaps you have sometimes felt small roundish hard bunches as big as a pea or a marble, under the skin at the side of the neck, under the arm pit, or in the groin. Sometimes these swell up and get sore. These nodules are the lymph glands. They surround the passages through the flesh along which the invading germs are likely to come, after they have burst through the skin. They are then like the fortifications along a road over which the enemy is likely to march. We can feel them only in certain places; but they are all over the body, under the skin, and beneath the membranes of the lungs and the digestive organs, wherever the body is most open to attack.

The garrison of these little forts are a peculiar sort of naked cells, which having no walls or covering to hold them in, can change their shape, reach out, and draw back again, as if they were sea-anemones or polyps or some other sort of water creature, and not part of our bodies at all. These cells are twenty or more times larger than the bacteria. And when the bacteria or other germs come along through the passages of the body on their way to the blood vessels, these guardian cells of ours reach out and grab them, drag them in, and devour them, like ogres that devour travelers who go by their castles, over the high road. Only in this case, our sympathy must be entirely on the side of the ogres.

But these lymph cells do not always stay in the lymph glands. When they like, they let go their anchorage, and go floating off through the blood, or through the lymph; for the lymph is only the watery part of the blood without the red corpuscles, which doesn’t stay in the blood vessels but soaks everywhere through the body, feeding and moistening the cells. These lymph cells, or white blood corpuscles, can go everywhere over the body, pushing and squeezing and elbowing their way among the other cells or traveling in the open spaces between them.

When the enemy enters the body, and begins to poison the blood, the white corpuscles smell them, and at once begin to crawl toward the smell, to reinforce their brothers in the particular lymph glands which happen to be at the point of attack. You get a sliver in your finger. The sliver opens a hole in your defense, and carries in some hostile germs. If, therefore, your lymph cells did not come to the rescue, you would inevitably die of blood poisoning. But they do come to the rescue. The white “matter” or “pus,” which in a few days comes out of the wound, is in large part the dead bodies of the fallen enemy and of the defenders who have perished in the fight. So in general, the invasion is stopped at the first lymph gland. A few thousand white corpuscles fall in the war. The enemy is massacred to the last man. And the body is saved.

Or else the foe get the upper hand; increase faster than the defenders can kill them off; break through the defense; enter the blood stream; invade the whole body; overrun the country; and put the inhabitants to the sword.

All our lives long, every day of our lives, we keep up this fight against disease. When measles does not threaten us, then it is colds. The game, therefore, is to strengthen our outer defenses, by keeping our food and houses clean, and our skins and teeth, and the air we breathe. And while we are doing that, we ought also to keep our bodies strong and resistant, with proper food and exercise and sleep, ready to put up a good fight if the enemy should break in. We know so much now about the causes of disease, that being sick or keeping well is a good deal under our own control.