What Bacteria do and Where They May be Found.--A walk through a crowded
city street on any warm day makes one fully alive to odors which pervade
the atmosphere. Some of these unpleasant odors, if traced, are found to
come from garbage pails, from piles of decaying fruit or vegetables, or
from some butcher shop in which decayed meat is allowed to stand. This
characteristic phenomena of decay is one of the numerous ways in which we
can detect the presence of bacteria. These tiny plants, "man's invisible
friends and foes," are to be found "anywhere, but not everywhere," in
nature. They swarm in stale milk, in impure water, in soil, in the living
bodies of plants and animals and in their dead bodies as well. Most
"catching" diseases we know to be caused directly by them; the processes of
decay, souring of milk, acid fermentation, the manufacture of nitrogen for
plants are directly or indirectly due to their presence. It will be the
purpose of the next paragraphs to find some of the places where bacteria
may be found and how we may know of their presence.
[Illustration: A steam sterilizer.]
How we catch Bacteria to Study Them.--To study bacteria it is first
necessary to find some material in which they will grow, then kill all
living matter in this food material by heating to boiling point (212 deg.) for
half an hour or more (this is called _sterilization_), and finally protect
the _culture medium_, as this food is called, from other living things that
might grow upon it.
One material in which bacteria seem to thrive is a mixture of beef extract,
digested protein and gelatine or agar-agar, the latter a preparation
derived from seaweed. This mixture, after sterilization, is poured into
flat dishes with loose-fitting covers. These _petri_ dishes, so called
after their inventor, are the traps in which we collect and study bacteria.
Where Bacteria might Grow.--Expose a number of these sterilized dishes,
each for the same length of time, to some of the following conditions:
(_a_) exposed to the air of the schoolroom.
(_b_) exposed in the halls of the school while pupils are passing.
(_c_) exposed in the halls of the school when pupils are not moving.
(_d_) exposed at the level of a dirty and much-used city street.
(_e_) exposed at the level of a well-swept and little-used city street.
(_f_) exposed in a city park.
(_g_) exposed in a factory building.
(_h_) dirt from hands placed in dish.
(_i_) rub interior of mouth with finger and touch surface of dish.
(_j_) touch surface of dish with decayed vegetable or meat.
(_k_) touch surface of dish with dirty coin or bill.
(_l_) place in dish two or three hairs from boy's head.
This list might be prolonged indefinitely.
[Illustration: Colonies of bacteria growing in a petri dish.]
Now let us place all of the dishes together in a moderately warm place (a
closet in the schoolroom will do) and watch for results. After a day or two
little spots, brown, yellow, white, or red, will begin to appear. These
spots, which grow larger day by day, are _colonies_ made up of millions of
bacteria. But probably each colony arose from a single bacterium which got
into the dish when it was exposed to the air.
How we may isolate Bacteria of Certain Kinds from Others.--In order to get
a number of bacteria of a given kind to study, it becomes necessary to grow
them in what is known as a pure culture. This is done by first growing the
bacteria in some medium such as beef broth, gelatin, or on potato.[20] Then
as growth follows the colonies of bacteria appear in the culture media or
the beef broth becomes cloudy. If now we wish to study one given form, it
becomes necessary to isolate them from the others. This is done by the
following process: a platinum needle is first passed through a flame to
_sterilize_ it; that is, to kill all living things that may be on the
needle point. Then the needle, which cools very quickly, is dipped in a
colony containing the bacteria we wish to study. This mass of bacteria is
quickly transferred to another sterilized plate, and this plate is
immediately covered to prevent any other forms of bacteria from entering.
When we have succeeded in isolating a certain kind of bacterium in a given
dish, we are said to have a _pure culture_. Having obtained a pure culture
of bacteria, they may easily be studied under the compound microscope.
Footnote 20: For directions for making a culture medium, see
Hunter, _Laboratory Problems in Civic Biology_. Culture
tubes may be obtained, already prepared, from Parke, Davis,
and Company or other good chemists.
[Illustration: A pure culture of bacteria. Notice that the bacteria are all
the same size and shape.]
Size and Form.--In size, bacteria are the most minute plants known. A
bacterium of average size is about 1/10000 of an inch in length, and
perhaps 1/50000 of an inch in diameter. Some species are much larger,
others smaller. A common spherical form is 1/50000 of an inch in diameter.
They are so small that several million are often found in a single drop of
impure water or sour milk. Three well-defined forms of bacteria are
recognized: a spherical form called a _coccus_, a rod-shaped bacterium, the
_bacillus_, and a spiral form, the _spirillum_. Some bacteria are capable
of movement when living in a fluid. Such movement is caused by tiny
lashlike threads of protoplasm called _flagella_. The flagella project from
the body, and by a rapid movement cause locomotion to take place. Bacteria
reproduce with almost incredible rapidity. It is estimated that a single
bacterium, by a process of division called _fission_, will give rise to
over 16,700,000 others in twenty-four hours. Under unfavorable conditions
they stop dividing and form rounded bodies called spores. This spore is
usually protected by a wall and may withstand very unfavorable conditions
of dryness or heat; even boiling for several minutes will not kill some
forms.
[Illustration: A figure to show the relative size and shape of (1) a green
mold, (2) yeast cells, and (3) different forms of bacteria; _B_, bacillus;
_C_, coccus; _S_, spirillum forms. The yeast and bacteria are drawn to
scale, they are much enlarged in proportion to the green mold, being
actually much smaller than the mold spores seen at the top of the picture.]
Where Bacteria are most Numerous.--As the result of our experiments, we can
make some generalizations concerning the presence of bacteria in our own
environment. They are evidently present in the air, and in greater quantity
in air that is moving than quiet air. Why? That they stick to particles of
dust can be proven by placing a little dust from the schoolroom in a
culture dish. Bacteria are present in greater numbers where crowds of
people live and move, the air from dusty streets of a populous city
contains many more bacteria than does the air of a village street. The air
of a city park contains relatively few bacteria as compared with the
near-by street. The air of the woods or high mountains fewer still. Why?
Our previous experiment has shown that dirt on our hands, the mouth and
teeth, decayed meat and vegetables, dirty money, the very hairs of our head
are all carriers of bacteria.
Fluids the Favorite Home of Bacteria.--Tap water, standing water, milk,
vinegar, wine, cider all can be proven to contain bacteria by experiments
similar to those quoted above. Spring or artesian well water would have
very few, if any, bacteria, while the same quantity of river water, if it
held any sewage, might contain untold millions of these little organisms.
Foods preferred by Bacteria.--If bacteria are living and contain no
chlorophyll, we should expect them to obtain protein food in order to grow.
Such is not always the case, for some bacteria seem to be able to build up
protein out of simple inorganic nitrogenous substances. If, however, we
take several food substances, some containing much protein and others not
so much, we will find that the bacteria cause decay in the proteins almost
at once, while other food substances are not always attacked by them.
[Illustration: Growth of bacteria in a drop of impure water allowed to run
down a sterilized culture in a dish.]
What Bacteria do to Foods.--When bacteria feed upon a protein they use part
of the materials in the food so that it falls to pieces and eventually
rots. The material left behind after the bacteria have finished their meal
is quite different from its original form. It is broken down by the action
of the bacteria into gases, fluids, and some solids. It has a
characteristic "rotten" odor and it has in it poisons which come as a
result of the work of the bacteria. These poisonous wastes, called
_ptomaines_, we shall learn more about later.
Conditions Favorable and Unfavorable to the Growth of Bacteria.--Moisture
and Dryness.--_Experiment_.--Take two beans, remove the skins, crush one,
soak the second bean overnight and then crush it. Place in test tubes, one
dry, the second with water. Leave in a warm place two or three days, then
smell each tube. In which is decay taking place? In which tube are bacteria
at work? How do you know?
Moisture.--Moisture is an absolute need for bacterial growth, consequently
keeping material dry will prevent the growth of germs upon its surface.
Foods, in order to decay, must contain enough water to make them moist.
Bacteria grow most freely in fluids.
Light.--If we cover one half of a petri dish in which bacteria are growing
with black paper and then place the dish in a light warm place for a few
days, the growth of bacteria in the light part of the dish will be found to
be checked, while growth continues in the covered part. It is a matter of
common knowledge that disease germs thrive where dirt and darkness exist
and are killed by any long exposure to sunlight. This shows us the need of
light in our homes, especially in our bedrooms.
Air.--We have seen that plants need oxygen in order to perform the work
that they do. This is equally true of all animals. But not all bacteria
need _air_ to live; in fact, some are killed by the presence of air. Just
how these organisms get the oxygen necessary to oxidize their food is not
well understood. The fact that some bacteria grow without air makes it
necessary for us to use the one sure weapon we have for their
extermination, and that is heat.
Heat.--_Experiment._--Take four cultures containing bouillon, inoculate
each tube with bacteria and plug each tube with absorbent cotton. Place one
tube in the ice box, a second tube in a dark closet at a moderate
temperature, a third in a warm place (about 100 deg. Fahrenheit), and boil the
contents of the fourth tube for ten minutes, then place it with tube number
two. In which tubes does growth take place most rapidly? Why?
Bacteria grow very slowly if at all in the temperature of an ice box, very
rapidly at the room temperature of from 70 deg. to 90 deg. and much less rapidly at
a higher temperature. All bacteria except those which have formed spores
can be instantly killed as soon as boiling point is reached, and most
spores are killed by a few minutes boiling.
Sterilization.--The practical lessons drawn from _sterilization_ are many.
We know enough now to boil our drinking water if we are uncertain of its
purity; we sterilize any foods that we believe might harbor bacteria, and
thus keep them from spoiling. The industry of canning is built upon the
principle of sterilization.
Canning.--Canning is simply a method by which first the bacteria in a
substance are killed by heating and then the substance is put into vessels
into which no more bacteria may gain entrance. This is usually done at home
by boiling the fruit or vegetable to be canned either in salt and water or
with sugar and water, either of which substances aids in preventing the
growth of bacteria. The time of boiling will be long or short, depending
upon the materials to be canned. Some vegetables, as peas, beans, and corn,
are very difficult to can, probably because of spores of bacteria which may
be attached to them. Fruits, on the other hand, are usually much easier to
preserve. After boiling for the proper time, the food, now free from all
bacteria, must be put into jars or cans that are themselves absolutely
_sterile_ or free from germs. This is done by first boiling the jars, then
pouring the boiling hot material into the hot jars and sealing them so as
to prevent the entrance of bacteria later.
Uses of Canning.--Canning as an industry is of immense importance to
mankind. Not only does it provide him with fruits and vegetables at times
when he could not otherwise get them, but it also cheapens the cost of such
things. It prevents the waste of nature's products at a time when she is
most lavish with them, enabling man to store them and utilize them later.
Canning has completely changed the life of the sailor and the soldier, who
in former times used to suffer from various diseases caused by lack of a
proper balance of food.
[Illustration: Pasteurizing milk. Why should this be done?]
Pasteurization.--Milk is one of the most important food supplies of a great
city. It is also one of the most difficult supplies to get in good
condition. This is in part due to the fact that milk is produced at long
distances from the city and must be brought first from farms to the
railroads, then shipped by train, again taken to the milk supply depot by
wagon, there bottled, and again shipped by delivery wagons to the
consumers. When we remember that much of the milk used in New York City is
forty-eight hours old and when we realize that bacteria grow _very_ rapidly
in milk, we see the need of finding some way to protect the supply so as to
make it safe, particularly for babies and young children.
This is done by _pasteurization_, a method named after the French
bacteriologist Louis Pasteur. To pasteurize milk we heat it to a
temperature of not over 170 deg. Fahrenheit for from ten minutes to half an
hour. By such a process all harmful germs will be killed and the keeping
qualities of the milk greatly lengthened. Most large milk companies
pasteurize their city supply by a rapid pasteurization at a much higher
temperature, but this method slightly changes the flavor of the milk.
Cold Storage.--Man has also come to use cold to keep bacteria from growing
in foods. The ice box at home and cold storage on a larger scale enables
one to keep foods for a more or less lengthy period. If food is frozen, as
in cold storage, it might keep without growth of bacteria for years. But
fruits and vegetables cannot be frozen without spoiling their flavor. And
all foods after freezing seem particularly susceptible to the bacteria of
decay. For that reason products taken from cold storage must be used at
once.
Ptomaines.--Many foods get their flavor from the growth of molds or
bacteria in them. Cheese, butter, the gamey taste of certain meats, the
flavor of sauerkraut, are all due to the work of bacteria. But if bacteria
are allowed to grow so as to become very numerous, the ptomaines which
result from their growth in foods may poison the person eating such foods.
Frequently ptomaine poisoning occurs in the summer time because of the
rapid growth of bacteria. Much of the indigestion and diarrhoea which
attack people during the summer is doubtless due to this kind of poisoning.
Preservatives.[21]--This leads us to ask if we may not preserve food in
ways other than those mentioned so as to protect ourselves from danger of
ptomaine poisoning. Many substances check the development of bacteria and
in this way they _preserve_ the food. Preservatives are of two kinds, those
harmless to man and those that are poisonous. Of the former, salt and sugar
are examples; of the latter, formaldehyde and possibly benzoic acid.
Footnote 21: Perform experiment here to determine the value
of different preservatives. Use sugar, salt, vinegar,
boracic acid, benzoic acid, formaldehyde, and alcohol.
Sugar.--We have noted the use of sugar in canning. Small amounts of sugar
will be readily attacked by yeasts, molds, and bacteria, but a 40 to 50 per
cent solution will effectually keep out bacteria. Preserves are fruits
boiled in about their own weight of sugar. Condensed milk is preserved by
the sugar added to it; so are candied and, in part, dried fruits.
Salt.--Salt has been used for centuries to keep foods. Meats are smoked,
dried, and salted; some are put down in strong salt solutions. Fish,
especially cod and herring, are dried and salted. The keeping of butter is
also due to the salt mixed with it. Vinegar is another preservative. It,
like salt, changes the flavor of materials kept in it and so cannot come
into wide use. Spices are also used as preservatives.
Harmful Preservatives.--Certain chemicals and drugs, used as preservatives,
seem to be on the border line of harmfulness. Such are benzoic acid, borax,
or boracic acid. Such drugs _may_ be harmless in small quantities, but
unfortunately in canned goods we do not always know the amount used. The
national government in 1906 passed what is known as the Pure Food Law,
which makes it illegal to use any of these preservatives (excepting benzoic
acid in very small amounts). Food which contains this preservative will be
so labeled and should not be given to children or people with weak
digestion. Unfortunately people do not always read the labels and thus the
pure food law is ineffective in its working. Infrequently formaldehyde or
other preservatives are used in milk. Such treatment renders milk unfit for
ordinary use and is an illegal process.
Disinfectants.[22]--Frequently it becomes necessary to destroy bacteria
which cause diseases of various kinds. This process is called
_disinfecting_. The substances commonly used are carbolic acid, formalin or
formaldehyde, lysol, and bichloride of mercury. Of these, the last named is
the most powerful as well as the most dangerous to use. As it attacks
metal, it should not be used in a metal pail or dish. It is commonly put up
in tablets which are mixed to form a 1 to 1000 solution. Such tablets
should be carefully safeguarded because of possible accidental poisoning.
Footnote 22: Experiment to determine the most effective
disinfectants. Use tubes of bouillon containing different
strength solutions of formaldehyde, lysol, iodine, carbolic
acid, and bichloride of mercury. Results. Conclusions.
Formaldehyde used in liquid form is an excellent disinfectant. When burned
in a formalin candle, it sets free an intensely pungent gas which is often
used for disinfecting sick rooms after the patient has been removed.
Carbolic acid is perhaps the best disinfectant of all. If used in a
solution of about 1 part to 25 of water, it will not burn the skin. It is
of particular value to disinfect skin wounds, as it heals as well as
cleanses when used in a weak solution. Its rather pleasant odor makes it
useful to cover up unpleasant smells of the sick room.
The fumes of burning sulphur, which are so often used for disinfecting, are
of little real value.
[Illustration: This shows how organic matter is broken down by bacteria so
it may be used again by green plants.]
Bacteria cause Decay.--Let us next see in what ways the bacteria directly
influence man upon the earth. Have you ever stopped to consider what life
would be like on the earth if things did not decay? The sea would soon be
filled and the land covered with dead bodies of plants and animals.
Conditions of life would become impossible and living things on the earth
would cease to exist.
Fortunately, bacteria cause decay. All organic matter, in whatever form, is
sooner or later decomposed by the action of untold millions of bacteria
which live in the air, water, and soil. These soil bacteria are most
numerous in rich damp soils containing large amounts of organic material.
They are very numerous around and in the dead bodies of plants and animals.
To a considerable degree, then, these bacteria are useful in feeding upon
these dead bodies, which otherwise would soon cover the surface of the
earth to the exclusion of everything else. Bacteria may thus be scavengers.
They oxidize organic materials, changing them to compounds that can be
absorbed by plants and used in building protoplasm. Without bacteria and
fungi it would be impossible for life to exist on the earth, for green
plants would be unable to get the raw food materials in forms that could be
used in making food and living matter. In this respect bacteria are of the
greatest service to mankind.
[Illustration: Microscopic appearance of ordinary milk, showing fat
globules and bacteria which cause the souring of milk.]
Relation to Fermentation.--They may incidentally, as a result of this
process of decay, continue the process of fermentation begun by the yeasts.
In making vinegar the yeasts first make alcohol (see page 135) which the
bacteria change to acetic acid. The lactic acid bacteria, which sour milk,
changing the milk sugar to an acid, grow very rapidly in a warm
temperature; hence milk which is cooled immediately and kept cool or which
is pasteurized and kept in a cool place will not sour readily. Why? These
same lactic acid bacteria may be useful when they sour the milk for the
cheese maker.
Other Useful Bacteria.--Certain bacteria give flavor to cheese and butter,
while still other bacteria aid in the "curing" of tobacco, in the
production of the dye indigo, in the preparation of certain fibers of
plants for the market, as hemp, flax, etc., in the rotting of animal matter
from the skeletons of sponges, and in the process of tanning hides to make
leather.
[Illustration: A field of alfalfa, a plant which harbors the
nitrogen-fixing bacteria.]
Nitrogen-fixing Bacteria.--Still other bacteria, as we have seen before,
"change over" nitrogen in organic material in the soil and even the free
nitrogen of the air so that it can be used by plants in the form of a
compound of nitrogen. The bacteria living in tubercles on the roots of
clover, beans, peas, etc., have the power of thus "fixing" the free
nitrogen in the air found between particles of soil. This fact is made use
of by farmers who rotate their crops, growing first a crop of clover or
other plants having root tubercles, which produce the bacteria, then
plowing these in and planting another crop, as wheat or corn, on the same
area. The latter plants, making use of the nitrogen compounds there,
produce a larger crop than when grown in ground containing less nitrogenous
material.
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.
[Illustration: Deaths from tuberculosis compared with other contagious
diseases in the city of New York in 1908.]
Tuberculosis.--The one disease responsible for the greatest number of
deaths--perhaps one seventh of the total on the globe--is tuberculosis. It
is estimated that of all people alive in the United States to-day,
5,000,000 will die of this disease. But this disease is slowly but surely
being overcome. It is believed that within perhaps one hundred years, with
the aid of good laws and sanitary living, it will be almost extinct.
[Illustration: This curve shows a decreasing death rate from tuberculosis.
Explain.]
Tuberculosis is caused by the growth of bacteria, called the _tubercle
bacilli_, within the lungs or other tissues of the human body. Here they
form little tubers full of germs, which close up the delicate air passages
in the lungs, while in other tissues they give rise to hip-joint disease,
scrofula, lupus, and other diseases, depending on the part of the body they
attack. Tuberculosis may be contracted by taking the bacteria into the
throat or lungs or possibly by eating meat or drinking milk from tubercular
cattle. Especially is it communicated from a consumptive to a well person
by kissing, by drinking or eating from the same cup or plate, using the
same towels, or in coming in direct contact with the person having the
germs in his body. Although there are always some of the germs in the air
of an ordinary city street, and though we may take some of these germs into
our bodies at any time, yet the bacteria seem able to gain a foothold only
under certain conditions. It is only when the tissues are in a worn-out
condition, when we are "run down," as we say, that the parasite may obtain
a foothold in the lungs. Even if the disease gets a foothold, it is quite
possible to cure it if it is taken in time. The germ of tuberculosis is
killed by exposure to bright sunlight and fresh air. Thus the course of the
disease may be arrested, and a permanent cure brought about, by a life in
the open air, the patient sleeping out of doors, taking plenty of
nourishing food and very little exercise. See also Chapter XXIV.
[Illustration: This figure shows how sewage from a cesspool (_c_) might get
into the water supply: _lm_, layer of rock; _w_, wash water.]
Typhoid Fever.--One of the most common germ diseases in this country and
Europe is typhoid fever. This is a disease which is conveyed by means of
water and food, especially milk, oysters, and uncooked vegetables. Typhoid
fever germs live in the intestine and from there get into the blood and are
carried to all parts of the body. A poison which they give off causes the
fever so characteristic of the disease. The germs multiply very rapidly in
the intestine and are passed off from the body with the excreta from the
food tube. If these germs get into the water supply of a town, an epidemic
of typhoid will result. Among the recent epidemics caused by the use of
water containing typhoid germs have been those in Butler, Pa., where 1364
persons were made ill; Ithaca, N. Y., with 1350 cases; and Watertown, N.
Y., where over 5000 cases occurred. Another source of infection is milk.
Frequently epidemics have occurred which were confined to users of milk
from a certain dairy. Upon investigation it was found that a case of
typhoid had occurred on the farm where the milk came from, that the germs
had washed into the well, and that this water was used to wash the milk
cans. Once in the milk, the bacteria multiplied rapidly, so that the
milkman gave out cultures of typhoid in his milk bottles. Proper
safeguarding of our water and milk supply is necessary if we are to keep
typhoid away.
Blood Poisoning.--The bacterium causing blood poisoning is another
toxin-forming germ. It lives in dust and dirt and is often found on the
skin. It enters the body through cuts or bruises. It seems to thrive best
in less oxygen than is found in the air. It is therefore important not to
close up with court-plaster wounds which such germs may have entered. It,
with typhoid, is responsible for four times as many deaths as bullets and
shells in time of battle. The wonderfully small death rate of the Japanese
army in their war with Russia was due to the fact that the Japanese
soldiers always boiled their drinking water before using it, and their
surgeons always dressed all wounds on the battlefield, using powerful
antiseptics in order to kill any bacteria that might have lodged in the
exposed wounds.
[Illustration: This figure shows how a milk route might be instrumental in
spreading diphtheria. _X_ is a farm on which a case of diphtheria occurred
that was responsible for all the cases along milk routes _A_ and _F_ in
Hyde Park, Dorchester, and Milton. How would you explain this?]
Other Diseases.--Many other diseases have been traced to bacteria.
Diphtheria is one of the best known. As it is a throat disease, it may
easily be conveyed from one person to another by kissing, putting into the
mouth objects which have come in contact with the mouth of the patient, or
by food into which the germs have been carried. Another disease which
probably causes more misery in the world than any other germ disease is
syphilis. Hundreds of thousands of new-born babies die annually or grow up
handicapped by deformities from this dread scourge. Syphilis and gonorrhea,
both diseases of the same sort and contracted in the same manner, hand down
to innocent wives and still more innocent children a heritage of disease
"even unto the third and fourth generation." Grippe, pneumonia, whooping
cough, and colds are believed to be caused by bacteria. Other diseases, as
malaria, yellow fever, sleeping sickness, and probably smallpox, scarlet
fever, and measles, are due to the attack of one-celled animal parasites.
Of these we shall learn later in Chapter XV.
Immunity.--It has been found that after an attack of a germ disease the
body will not soon be again attacked by the same disease. This immunity, of
which we will learn more later, seems to be due to a manufacture in the
blood of substances which fight the bacteria or their poisons. If a person
keeps his body in good physical condition and lives carefully, he will do
much toward acquiring this natural immunity.
Acquired Immunity.--Modern medicine has discovered means of protecting the
body from some contagious diseases. Vaccination as protection against
smallpox, the use of antitoxins (of which more later) against diphtheria,
and inoculation against typhoid are all ways in which we may be protected
against diseases.
Methods of fighting Germ Diseases.--As we have seen, diseases produced by
bacteria may be caused by the bacteria being _directly_ transferred from
one person to another, or the disease may obtain a foothold in the body
from food, water, or by taking them into the blood through a cut or a wound
or a body opening.
It is evident that as individuals we may each do something to prevent the
spread of germ diseases, especially in our homes. We may keep our bodies,
especially our hands and faces, clean. Sweeping and dusting may be done
with damp cloths so as not to raise a dust; our milk and water, when from a
suspicious supply, may be _sterilized_ or pasteurized. Wounds through which
bacteria might obtain foothold in the body should be washed with some
_antiseptic_ such as carbolic acid (1 part to 25 water), which kills the
germs. In a later chapter we shall learn more of how we may cooperate with
the authorities to combat disease and make our city or town a better place
in which to live.[23]
Footnote 23: Teachers may take up parts or all of Chapter
XXIV at this point. I have found it advisable to repeat much
of the work on bacteria _after_ the students have taken up
the study of the human organism.