Bacteria, or microbes, the smallest living things, are visible only under a microscope of high power. (See “Plant Biology,” p. 182.) They obtain food either from dead tissue or from degenerate tissue of living plants and animals. The green plants and the animals now upon the earth have proved their fitness to survive by successfully resisting these one-celled vegetable germs, or bacteria. Microbe diseases attack only the weaker individuals of the human species, or those who have gone to regions where there are microbes which their bodies have not yet acquired the power of resisting.

Usefulness of Bacteria.—Their chief work is to destroy dead tissue and return it to the soil and air for the use of green plants again, otherwise the earth would be filled with carcasses, etc. They are indispensable in soil formation. They give the agreeable flavors to butter and cheese, and cause milk to sour. A rod-shaped bacterium is called a bacillus (Fig. 130); a spherical one is a coccus.

Multiplication of Bacteria.—This is by division or fission. Sometimes, instead of dividing, a little rounded mass known as a spore appears. The spore breaks out and the bacterium itself perishes. Species which do not produce spores are readily destroyed, but spores have a hard, tough shell, and they may be dried or heated even to boiling without being killed. Spores float through the air and start new colonies. Most common bacteria grow best between 70° and 95° F. They render it difficult to preserve foods, especially proteid foods (cheese, lean meat, eggs, etc.). Food decays slowly if at all below 70° and above 125°. Direct sunlight, or the temperature of boiling water (212° F.) kills bacteria but not spores. Pantries, kitchen, and sickrooms should have bright walls and all the light possible. Boiling water should be poured into the sink, and dish cloths should be thoroughly washed in boiling water.

Diseases due to Bacteria.—A germ disease is usually due partly or wholly to substances called toxins produced by the bacteria. Most disease germs attack a single organ of the body. Diphtheria is caused by a species (Fig. 130) that grows on the mucous membrane of the throat; this germ produces a powerful toxin. The germs of typhoid fever (Fig. 131) and Asiatic cholera multiply in the small intestine. In both these diseases the source of infection is the diarrhœal discharges from the alimentary canal. Flies may carry the germs on their feet from the discharge to food. Sometimes typhoid fever cases occur throughout a town because the water supply has become contaminated by sewage. Cases may occur only in families that buy milk from a certain dairy, because the milk cans have been washed in contaminated water. In caring for a typhoid patient all suspicious material should be disinfected or burned. Germs of tuberculosis (called consumption if the disease is in the lungs) may float through the air. Recent investigations indicate, however, that infection usually occurs through the alimentary canal, the germs being swallowed, then absorbed and taken to the lungs in the blood or lymph. To prevent a patient from reinfecting himself in new parts of the lungs or elsewhere, he should carefully cleanse his teeth, mouth, and throat (by gargling with formal or lysol) before eating.

Mosquito Fevers.—Malaria, yellow fever, and probably dengue are transmitted each by a different genus of mosquito (Fig. 132). A mosquito of the malarial genus may bite a patient and suck into its body blood-corpuscles containing spores of the malarial parasite (a protozoan animal, see “Animal Biology,” p. 7). Afterwards a spore (in another stage) may be transmitted by this mosquito when it bites another person. The germ enters a red corpuscle, grows, and finally divides into many little spores. At this moment the corpuscle itself breaks up, setting free in the blood the spores and toxin formed. This causes the chill and fever. This development usually takes forty-eight hours, hence the fever occurs every other day. These mosquitoes begin to fly at dusk. How are they recognized? (Fig. 132.) They should be kept out of houses by screens or from the beds by netting. Kerosene should be poured on breeding places at the rate of one ounce for fifteen square feet of standing water. This should be repeated twice a month. Cactus macerated in water may be used, and forms a permanent film on the water. Stagnant pools may be filled or drained (Exp. 4). Malarial patients should themselves be screened, as the chief source of danger to others; for only mosquitoes who suck the blood of malarial patients will transmit the disease. Even then it is only transmitted to those whose white blood corpuscles are unable to protect them (Fig. 133).

Further Means of Protection against Disease Germs.—The best protection is physical vigor. There are certain substances called opsonins which exist in the plasma of the blood of disease-resisting persons; these opsonins give the white corpuscles the power to devour disease germs. The serum of the blood also develops antitoxins which neutralize the toxins formed in disease. Not only can the white corpuscles and serum kill bacteria, but most of the secretions of the healthy body (gastric juice, nasal secretions, etc.) are bacteria-killing as well. Persons in a low state of health most readily succumb to disease. Excess in eating may lessen the germicidal power of gastric juice and inactivity that of the lymph. The same germ disease does not usually attack the same person twice, as the body becomes immune; that is, an opsonin, or an antitoxin, is developed which cures the first attack and remains to protect the body in future.

The periods of quarantine or isolation for several common germ diseases are given in the following table:—

Water Supply.—Bacteria are more abundant in flowing streams than in water standing in lakes or reservoirs (contrary to the usual belief). They are most abundant in rivers that flow through populous regions. They are comparatively scarce in dry, sandy soils, and very numerous in moist, loamy soils. The water of cities should never be taken from a stream or lake into which sewerage flows unless it is thoroughly filtered. Filters are constructed thus: first a layer of small stones, next a layer of coarse sand, lastly a layer of very fine sand on top, the total thickness being four or five feet. Beneficial microbes live upon the grains of sand and destroy all, or nearly all, of the dangerous microbes as the water slowly soaks through. The construction of such waterworks is left to sanitary engineers, of course, and the average citizen does not need to know the details.

The department of street cleaning should receive the willing coöperation of all citizens. Banana peelings, paper, etc., should not be thrown upon the street or school grounds. Garbage, ashes, and rubbish should be placed in separate cans, as the rules provide. Garbage cans, if not thoroughly cleaned, acquire unpleasant odors and breed flies and bacteria. They should be thoroughly washed with very hot water and sal soda and scalded with boiling water and scrubbed with an old broom.[11]

The chief duties of the Health Department are: quarantine isolation and disinfection, with the purpose of preventing or controlling contagious and infectious diseases; inspection of dairies, slaughterhouses, and other sanitary work; inspection of milk[12] and other food stuffs; the department gathers vital statistics; it enforces the rules for disinfection of public buildings.

Importance of Coöperation with the Health Department.—Only an ignorant and short-sighted person would fail to coöperate promptly and cheerfully with local or state health officers. It is for the benefit and protection of every one that the truth concerning contagious diseases be reported promptly. Only in this way may outbreaks of disease be prevented and many lives saved. He is a bad citizen and a public enemy who will conceal a case of disease dangerous to the community. Outbreaks of fatal diseases may be easily prevented or stamped out if the health officer is sustained and his directions carried out.