American Red Cross Text-Book on Home Hygiene and Care of the Sick. American National Red Cross
of bacilli, those causing pneumonia are cocci, while those causing Asiatic cholera are spirilla.
Size.
—Bacteria vary greatly in size. Average rod-shaped bacteria are about 1/25000 of an inch long, but there are undoubtedly organisms so small that they cannot be seen, even by means of the strongest microscopes we now possess.
Motion.
—The power of motion in certain species of bacteria is due to hair-like appendages called flagella. These flagella by a lashing movement somewhat resembling the action of oars enable the organisms to move through fluids.
Multiplication.
—After bacteria have fully developed, each cell divides into two equal parts; the process of division is called fission. Each of these two parts rapidly grows into a full-sized organism. Then fission again takes place, so that four bacteria replace the original one. In each of the four, fission occurs again, and so the process of multiplication continues. As bacteria develop they group themselves in characteristic ways. Some, like the streptococci, arrange themselves in chains; the diplococci, in pairs; the tetrads, in groups of four; others in packets called sarcinæ, and still others, the staphylococci, form masses supposed to resemble bunches of grapes.
Under favorable conditions fission occurs rapidly; in some types a new generation may appear as often as every 15 minutes. Enormous multiplication would result if nothing occurred to check the process. But in nature such increase never continues unhindered, and bacteria, acting upon their food substances, produce acids and other materials injurious to themselves. Furthermore, lack of proper food, moisture, or favorable temperature, and competition with other organisms tend to prevent their unrestricted growth and multiplication.
Spores.
—Most bacteria die if conditions become unfavorable to their growth, but some enter into a resting stage. This stage is characterized by the development of round or oval glistening bodies called spores, which are of dense structure and possess an extraordinary power to withstand heat, chemicals, and unfavorable surroundings. Except in rare instances a single cell produces but one spore. As soon as favorable conditions of temperature, moisture, and food supply are restored, the spore develops into the active form of the germ; it may, however, remain dormant for months or years. Spore formation, however, occurs in only a very few varieties of pathogenic bacteria.
Distribution.
—Bacteria are very widely distributed in nature; they are in fact found practically everywhere on the surface of the earth. They are present in plants and water and food; on fabrics and furniture, walls and floors; and they are found in great numbers on the skin, hair, many mucous surfaces, and other tissues of the body.
PROTOZOA
The protozoa are the lowest group of the animal kingdom. Like bacteria they are composed of single cells so small as to be visible only under the microscope. They play an important part in causing certain diseases of man, especially in the tropics. Among the well-known human diseases of protozoan origin are malaria, amoebic dysentery, and sleeping-sickness. Protozoa also cause several wide-spread and serious plagues of domestic animals.
VISIBLE PARASITES
A few diseases are caused by parasites large enough to be seen with the naked eye. One of the most important is hookworm disease. This disease is caused by a tiny worm which penetrates the victim's skin and ultimately finds its way into the intestine. Other diseases also are caused by parasitic worms, such as tapeworms, pinworms, and trichinæ. The latter are acquired as a result of eating infected meat, particularly infected pork that has not been thoroughly cooked.
TRANSMISSION OF PATHOGENIC ORGANISMS
Pathogenic or disease producing organisms need for their development food, moisture, darkness, and warmth, conditions that exist within the human body. When one or more of these factors is unfavorable, development of germs is checked; if unfavorable conditions are extreme or long continued, the organisms begin to die. It is difficult to say at exactly what moment they will die if deprived of moisture or exposed to extremes of temperature or other unfavorable conditions, just as it would be impossible to state at exactly what moment a collection of house plants would all be dead if water were withheld, or if the room temperature were greatly reduced.
Most pathogenic organisms, however, do not flourish long outside the body, and owe their continued existence to a fairly direct transfer from person to person. They gain access to the body through mucous surfaces such as the respiratory and digestive tracts, and through breaks in the skin, such as cuts, abrasions, and the bites of certain insects. They leave the body chiefly in the nasal and mouth discharges, as in coughing, sneezing, and spitting, in the urine and bowel discharges, and in pus or "matter."
The problem of controlling communicable diseases, consequently, lies in preventing the bodily discharges of one person from travelling directly into the body of another. If a person is not expelling pathogenic germs, it is clear that he cannot pass diseases on to others. But both pathogenic and harmless germs follow the same routes from person to person, so that safety as well as decency lies in preventing so far as possible all exchanges of bodily discharges.
There are five routes by which the bodily discharges most frequently travel from one person to another. Four of these routes of infection are called public, because in most cases efforts of individuals alone are not sufficient to control them. The public routes are water, milk, food, and insects. The fifth, or private route, includes all means by which fresh discharges of one person are passed to another, as when nose and mouth discharges are carried in coughing, sneezing, and kissing, or when bowel and bladder discharges are carried by the hands. These five routes in a given case differ greatly in relative importance, but the fifth, or direct route plays an immense part, although its importance in causing sickness has only lately been recognized. It cannot be too strongly emphasized that the chief agent in the spread of human diseases is man himself, and the human hand is the great carrier of disease germs both to and from the body. If unclean hands could be kept away from the orifices of the body, particularly the mouth, many diseases would soon cease to exist.
Defenses of the Body
In