Bacteria cause Disease.--The most harmful bacteria are those which cause diseases of plants and animals. Certain diseases of plants--blights, rots, and wilts--are of bacterial nature. These do much annual damage to fruits and other parts of growing plants useful to man as food. But by far the most important are the bacteria which cause disease in man. They accomplish this by becoming parasites in the human body. Millions upon millions of bacteria exist in the human body at all times--in the mouth, on the teeth, in the blood, and especially in the lower part of the food tube. Some in the food tube are believed to be useful, some harmless, and some harmful; others in the mouth cause decay of the teeth, while a few kinds, if present in the body, may cause disease. [Illustration: Tubercles on the roots of the soy bean. They contain the nitrogen-fixing bacteria. (Fletcher's Soils.) Copyright by Doubleday, Page and Company.] It is known that bacteria, like other living things, feed and give off organic waste from _their own_ bodies. This waste, called a _toxin_, is poison to the host on which the bacteria live, and it is usually the production of this toxin that causes the symptoms of disease. Some forms, however, break down tissues and plug up the small blood vessels, thus causing disease. Diseases caused by Bacteria.--It is estimated that bacteria cause annually over 50 per cent of the deaths of the human race. As we will later see, a very large proportion of these diseases might be prevented if people were educated sufficiently to take the proper precautions to prevent their spread. These precautions might save the lives of some 3,000,000 of people yearly in Europe and America. Tuberculosis, typhoid fever, diphtheria, pneumonia, blood poisoning, syphilis, and a score of other germ diseases ought not to exist. A good deal more than half of the present misery of this world might be prevented and this earth made cleaner and better by the cooperation of the young people now growing up to be our future home makers. [Illustration: A single cell scraped from the roof of the mouth and highly magnified. The little dots are bacteria, most of which are harmless. Notice the comparative size of bacteria and cell.] How we take Germ Diseases.--Germ or contagious diseases either enter the body by way of the mouth, nose, or other body openings, or through a break in the skin. They may be carried by means of air, food, or water, but are usually _transmitted directly_ from the person who has the disease to a well person. This may be done through personal contact or by handling articles used by the sick person or by drinking or eating foods which have received some of the germs. From this it follows that if we know the methods by which a given disease is communicated, we may protect ourselves from it and aid the civic authorities in preventing its spread.