=Remember.= 1. Disease germs get into water from dirty places along the banks of the streams; they do not come from the ground. 2. Clear water is not always pure; germs do not make the water cloudy or muddy. 3. Wells often become infected by matter from closets seeping into them; make your closet water-tight. 4. Spring water is usually pure, but not all water that oozes out of the ground is spring water. CHAPTER XX TRANSMISSION OF DISEASE THROUGH THE AIR [Sidenote: Disease germs in the air] We take germs into our bodies with the air that we breathe. Since we cannot stop breathing and live, we must see to it that the air we breathe is kept pure. [Illustration: FIG. 47. (a) Prevalence of germs in air of thickly populated districts. (b) Prevalence of germs in air of sparsely populated districts.] There are always more germs in the air of places in which people live closely crowded together than where there are only a few people. This is proved by Figure 47, which shows that many more germs were found on a culture plate exposed in the downtown part of New York City than on another plate exposed far uptown, where there are not so many people. Remember, however, that all germs are not disease germs. [Sidenote: How we may keep disease germs out of the air:] How do the disease germs get into the air? When one sneezes, a spray of droplets is thrown into the air. If the person sneezing has the grip, these droplets contain the germs that cause grip. Whenever a person with consumption coughs, he sprays droplets which contain the germs that cause consumption. [Sidenote: (_1_) When coughing or sneezing] If a person would hold a handkerchief before his mouth when he coughs or sneezes, these droplets of moisture would not be sprayed into the air, and the disease germs in them would not be scattered about. You ask, "Shall everyone who is sick hold a handkerchief before the mouth when sneezing or coughing?" Everyone, whether sick or well, ought to hold a handkerchief before the mouth when sneezing or coughing. Learn to do this at once, and never forget it. [Sidenote: (_2_) When spitting] Another way by which disease germs get into the air is from the sputum. People spit on the floor or the sidewalk, and the sputum becomes dried; it is then blown about as dust. The germs of disease are not killed by drying, and when they get into our bodies with the dust which we breathe in, they immediately begin to grow. Disease germs get into the air chiefly through careless habits of coughing, sneezing, and spitting, and these careless habits can easily be prevented. [Sidenote: Why well people should not spit on floor or sidewalk]