In those good old times, every little while, whole cities would be smitten with a terrible disease called the plague. Perhaps you already know about the Great Plague of London in 1665, when seventy thousand people perished, and the dead lay in the streets because the living were too few to bury them. The same man who wrote the story of Robinson Crusoe, wrote also a story of this great plague, not a pleasant story, naturally, but one that you will want to read later when you are older.
The trouble was nothing in the world but dirt and rats. The rats lived in the dirt; and the minute, living plant that makes the plague, lived in the blood of the rats. From them it got into the blood of human beings. But so long as a city kept clean and free from rats, it never had the plague. But when it let itself get dirty, as ancient cities usually did, then it might lose a fifth of its inhabitants in a few months.
As people, therefore, began to be more decent, the plague began to disappear; and after about the time of our Revolutionary War, most of Europe had become so clean and civilized that they had no more plague there. But still it lingers in other parts of the world, where there is more dirt, and where people, instead of putting their waste tidily away in the bucket or burning it up, throw it out the back door for rats to eat. Always, even now, the plague threatens Asia. During the first ten years of this very civilized twentieth century five million persons died of it in India alone.
And all because of dirt and rats and fleas. The rat lives in the dirt. The fleas live on the rat, and when they bite the rat, get a stomachful of blood, and with it some five thousand or so of the little plants that cause the plague. Then the flea jumps off the rat, on to a man, and bites him. Then a few of these five thousand germs get into the man’s blood. By the next day, these few have become millions. Within a week, often within two days, the man is dead—simply poisoned. But if the man had kept his house clear of rats and his skin clear of fleas, by keeping them both clean, he would not have been poisoned at all.
I am sorry to say that since the year 1900, and even as late as 1909, there have been cases of the plague in one or two especially dirty cities in the United States. So the National Government had to interfere, to make them clean up and get rid of their rats. Otherwise we might have had a terrible time; while as it was, some three hundred people died—which is more human beings than most of us know by name.
But you can’t have the plague without rats, and you can’t have rats without dirt. So, therefore, every civilized government in the world keeps men at work in its seaports, killing the rats that come in the ships, lest they bring the plague from China or India, where they don’t mind a little dirt.
There is another animal, dirtier even than the rat, and on the whole rather more dangerous—and that is the fly. Wherever there is dirt, there are pretty sure to be the germs of various diseases. If there is anything the fly likes, it is dirt. He eats it; he wallows in it. The dirt sticks to his feet, and the disease germs stick to the dirt; for a fly is not nearly so much smaller than an elephant as a disease germ is smaller than a fly.
Then the fly tracks over our food or falls into our milk. He may carry a million germs on his body, and every time he puts down one of his six feet he plants at least one. In forty-eight hours this single one may have grown to sixteen thousand. Then some boy or girl eats the food and is sick; or some baby drinks the milk and dies.