Can you imagine who on earth is our human being's most dangerous enemy?—The rats! Men are the cleverest and most dangerous animals on earth. Rats come second. Men kill their enemies. They have killed millions of men since ancient times—but rats have killed hundreds of millions of men. Rats carry diseases. Black rats carry fleas, and these fleas give men bubonic plague. Bubonic plague still kills thousands of Asians and Africans today as it killed millions of Europeans in the past until brown rats came to Europe and pushed the black rats out. There is no plague in Europe today, but brown rats carry thirty-five other diseases typhus for example. Typhus has killed 200 million people in the last 400 years. Rats are dangerous. They not only carry diseases into men's homes, they also take men's food. They eat 33 million tons of grain every year. That is enough to feed 200 million people—so men die of hunger because rats eat their food. Rats are clever too. They have good leaders. Men try to poison them, but if their leaders tell them that food is dangerous they do not eat it. So our cities and villages are full of rats. We can go to the moon, but we cannot kill this dangerous enemy. Why? Is it because we are not clean? Human beings are afraid of rats mainly because ______.