SECONDARY, TERMINAL AND MIXED INFECTIONS.--THE EXTENSION OF INFECTION
IN THE INDIVIDUAL.--TUBERCULOSIS.--THE TUBERCLE BACILLUS.--FREQUENCY
OF THE DISEASE.--THE PRIMARY FOCI.--THE EXTENSION OF BACILLI.--THE
DISCHARGE OF BACILLI FROM THE BODY.--INFLUENCE OF THE SEAT OF DISEASE
ON THE DISCHARGE OF BACILLI.--THE INTESTINAL DISEASES.--MODES OF
INFECTION.--INFECTION BY SPUTUM SPRAY.--INFECTION OF WATER
SUPPLIES.--EXTENSION OF INFECTION BY INSECTS.--TRYPANOSOME
DISEASES.--SLEEPING SICKNESS.--MALARIA.--THE PART PLAYED BY
MOSQUITOES.--PARASITISM IN THE MOSQUITO.--INFECTION AS INFLUENCED BY
HABITS AND CUSTOMS.--HOOKWORM DISEASE.--INTER-RELATION BETWEEN HUMAN
AND ANIMAL DISEASES.--PLAGUE.--PART PLAYED BY RATS IN
TRANSMISSION.--THE PRESENT EPIDEMIC OF PLAGUE.
The infectious diseases are often complicated by secondary infections,
some other organism finding opportunity for invasion in the presence
of the injuries produced in the primary disease. In many diseases,
such as diphtheria, scarlet fever and smallpox, death is frequently
due to the secondary infection. The secondary invaders not only find
local conditions favoring a successful attack, but the activity of the
tissue cells on which the production of protective substances
essentially depends has suffered by the primary infection, or the
cells are occupied in meeting the exigencies of this. The body is in
the position of a state invaded by a second power where all its forces
and resources are engaged in repelling the first attack.
What are known as terminal infections occur shortly before death. No
matter what the disease which causes death, in the last hours of life
the body usually becomes invaded by organisms which find their
opportunity in the then defenceless tissues, and the end is often
hastened by this invasion.
There are also mixed infections in which two different organisms unite
in attack, each in some way assisting in the action of the other. The
best known example of this is in the highly infectious disease of
swine known as hog cholera. It has been shown that in this disease two
organisms are associated,--one an invisible and filterable organism,
and the other a bacillus. It was first supposed that the bacillus was
the specific organism; it was found in the lesions and certain, but
not all, the features of the disease were produced by inoculating hogs
with pure cultures. The disease so produced is not contagious, and the
contagious element seems to be due to the filterable virus.
The modes of transmission of infectious diseases are of great
importance and are the foundation of measures of public health. In the
preceding chapter we have seen that in the infected individual the
disease extends from one part of the body to another. There is a
primary focus of disease from which the extension takes place, and the
study of the modes of extension in the individual throws some light on
the much more difficult subject of the transmission of disease from
one individual to another. There are four ways by which extension in
the individual may take place.
1. By continuity of tissue, an adjoining tissue or organ becoming
infected by the extension of a focus of infection.
2. By means of lymphatics. Organisms easily enter these vessels which
are in continuity with the tissue spaces and receive the exudate from
the focus of infection. The organisms are carried to the lymph nodes,
which, acting as filters, retain them and for a time prevent a further
extension. The following illustrates the importance of the part the
nodes may play in mechanically holding back a flood of infection. A
physician examined after death the body of a person who died from
infection with a very virulent micrococcus and in the course of the
examination slightly scratched a finger. One of the organs of the body
was removed, sent to a laboratory and received by a laboratory worker,
a woman physician, who had slight abrasions and fissures in the skin
of the hands from contact with irritating chemicals. In the course of
a few hours the wound on the finger of the man became inflamed,
intensely painful, and red lines extended up the arm in the course of
the lymphatic vessels, showing that the organisms were in the
lymphatics and causing inflammation in their course. The lymph nodes
in the armpit into which these vessels empty became greatly inflamed,
swollen, and an abscess formed in them which was opened. There was
high fever, great prostration, a serious illness from which the man
did not recover for several months. The woman only handled the organ
which was sent to the laboratory in order to place it in a fluid for
preservation. She also had a focus of infection of a finger with the
same red lines on the arm, showing extension by the lymphatics; but
there was no halt of the infection in the armpit, for all the lymph
nodes there had been removed several years before in the course of an
operation for a tumor of the breast. A general infection of the blood
took place, there was very high fever, and death followed in a few
days. The halt of the infection is important in allowing time for the
body to make ready its means of defence. One cannot avoid comparing
the lymph node with a strong fortress thrown in the path of a
victorious invading army behind which the defenders may gather and
which affords them time to renovate their strength.
3. By means of the blood. The blood vessels are universally
distributed, the smaller vessels have thin walls easily ruptured and
easily penetrated. It is probable that in every infection some
organisms enter the blood which, under usual conditions, is peculiarly
hostile to bacteria. These may, however, be carried by the blood to
other organs and start foci of infection in these.
4. By means of continuous surfaces. The bacteria may either grow along
such surfaces forming a continuous or more or less broken layer, or
may be carried from place to place in the fluids which bathe them.
All these modes of extension are well shown in tuberculosis. This
disease is caused by a small bacillus which does not produce spores,
has no power of saphrophytic growth under natural conditions, and is
easily destroyed. Moisture and darkness are favorable conditions for
its existence, sunlight and dryness the reverse. There are three
varieties or strains of the tubercle bacilli which infect respectively
man, cattle and birds, and each class of animals shows considerable
resistance to the varieties of the bacillus which are most infectious
for the others.
The primary seat of the infection in man is generally in the upper
part of the lung. The organisms settle on the surface here and cause
multiplication of the cells and an inflammatory exudate in a small
area. With the continuous growth of the bacilli in the focus,
adjoining areas of the lung become affected, and there is further
extension in the immediate vicinity by means of the lymphatics. Small
nodules are formed and larger areas by their coalescence. Infection
with tuberculosis is so common that at least three-fourths of all
individuals over forty show evidences of it. The examination of two
hundred and twenty-five children of the average age of five years who
had died of diphtheria showed tuberculous infection in one-fifth of
the cases and the frequency of infection increases with age. The
defence on the part of the body is chiefly by the formation of dense
masses of cicatricial tissue which walls off the affected area and in
which the bacilli do not find favorable conditions for growth. This
mode of defence, which is probably combined with the production of
substances antagonistic to the toxines produced by the bacilli, is so
efficacious that in the great majority of cases no further extension
of the process takes place. In certain cases, however, the growth of
the bacilli in the focus is unchecked, the tissue about them is killed
and becomes converted into a soft semi-fluid material; further
extension then takes place. All parts of the enormous surface of the
lungs are connected by means of the system of air tubes or bronchi,
and the bacilli have favorable opportunity for distribution, which is
facilitated by sudden movements of the air currents in the lung
produced by coughing. The defence of the body can still keep pace with
the attack, and even in an advanced stage the infection can be checked
in some cases permanently; in others the check is but temporary, the
process of softening continues, and large cavities are produced by the
destruction of the tissue. On the inner surface of these cavities
there may be a rapid growth of bacilli.
From the lungs the bacilli are carried by the lymphatics to the lymph
nodes at the root of the lungs, in which a similar process takes
place; this, on the whole, is favorable, because further extension by
this route is for a time blocked. The extension by means of surfaces
continues, the abundant sputum which is formed in the lungs and which
contains large numbers of bacilli, becomes the vehicle of
transportation. The windpipe and larynx may become infected, the back
parts of each are more closely in contact with the sputum and are the
parts most generally infected. A large part of the sputum is swallowed
and infection of the intestine takes place, the lesions taking the
form of large ulcers. From the intestinal ulcers there is further
extension by means of the lymphatics, to the large lymph nodes in the
back of the abdominal cavity (Fig. 8-25); the bacilli may also pass
from the ulcers into the abdominal cavity and be distributed over the
surface of the peritoneum resulting in tuberculous peritonitis. When
the disease has reached an advanced stage, bacilli in small numbers
continually pass into the blood and are distributed by this over the
body, producing small nodules in many places. In rare cases
distribution by the blood is the principal method of extension, and
immense numbers of small foci of disease are produced, the form of
disease being known as acute miliary tuberculosis. Although the
bacilli are distributed everywhere, certain organs, as the brain and
muscles, are usually exempt, because in these the conditions are not
favorable to further growth of the bacilli. Tuberculosis, although
frequently a very acute disease, is usually one of the best types of a
chronic disease and may last for many years. The chronic form is
characterized by periods of slow or rapid advance when conditions
arise in the body favorable for the growth of the bacilli, and periods
when the disease is checked and quiescent, the defensive forces of the
body having gained the upper hand. Often the intervention of some
other disease so weakens the defences of the body that the bacilli
again find their opportunity. Thus typhoid fever, scarlet fever and
other diseases may be followed by a rapidly fatal advance of the
tuberculosis, starting from some old and quiescent focus of the
disease.
Tuberculosis is also one of the best examples of what is known as
latent infection. In this the infectious organisms enter the body and
produce primary lesions in which the organisms persist but do not
extend owing to their being enclosed in a dense and resistant tissue,
or to the production of a local immunity to their action. Dr. Head has
recently examined the children of households in which there was open
tuberculosis in some member of the household. By open tuberculosis is
understood a case from which bacilli are being discharged. He found
with scarcely an exception that all the children in such families
showed evidences of infection. The detection of slight degrees of
tuberculous infection is now made easy by certain skin reactions on
inoculation of the skin with a substance derived from the tubercle
bacilli. Such latent infections may never become active and in the
majority of cases do not. When, however, in consequence of some
intercurrent disease or conditions of malnutrition the general
defences of the body become weakened extension follows. Such latent
infections explain the enormous frequency of tuberculosis in prisons.
Under the general prison conditions infection in the prisons probably
does not take place to any extent, and the disease is as common when
the prisoners are kept in individual cells as in common prisons. It is
probable that in these cases the prisoners have latent tuberculosis
when entering, and the disease becomes active under the moral and
physical depression which prison life entails.
For the extension of infection from one individual to another the
infecting organisms must in some way be transferred. The most
important of the conditions influencing this are the localization of
the disease and the character of the infectious organisms,
particularly with regard to their resistance to the conditions met
with outside of the body. The seat of disease influences the discharge
of organisms; thus, if the disease involve any of the surfaces the
organisms become mingled with the secretions of the surface and are
discharged with these. If the seat of disease be in the lungs, the
throat or the mouth, the sputum forms the medium of extension, which
can take place in many ways. The sputum may become dried, forms part
of the dust and the organisms enter with the inspired air. The
organisms which cause most of the diseases in which the sputum becomes
infectious are quickly destroyed by conditions in the open, such as
the sunlight and drying; street dust does not play so prominent a part
in extension as is generally supposed. Organisms find much more
favorable conditions within houses. It is now generally recognized
that infection with tuberculosis does not take place in the open, but
in houses in which the bacilli on being discharged are not destroyed.
The hands, the clothing and surroundings even with the exercise of the
greatest care may become soiled with the saliva.
It has been shown that in coughing and speaking very fine particles of
spray are formed by the intermingling of air and saliva, which may be
projected a considerable distance and remain floating in the air for
some time. These particles are so fine as to be invisible; they may be
inspired, and their presence in the air forms an area of indeterminate
extent around the infected person within which such infection is
possible. Such spray formation is also an important means of the
extension of infection in the sick individual, for it is continually
formed and inspired. It is in this way that the extreme prevalence of
broncho-pneumonia in infants and young children is to be explained. No
matter what the essential disease, an almost constant finding in young
children after death is small areas of inflammation in the lungs in
and around the terminations of the air tubes. The situation renders it
evident that the organisms which caused the lesions entered the lung
by the air tubes. The mouth of the child is unclean and harbors
numbers of the same sort of organisms as those causing the lung
inflammation; but in the absence of such a mode of infection as is
given by spray formation it is difficult to see how the extension from
the mouth to the lungs could take place. The weakened condition of the
body in these cases favors the secondary infection.
If the disease be located in the intestines, as in typhoid fever and
dysentery, the organisms are contained in the fecal discharges, and by
means of these the infection is extended. In typhoid fever, dysentery
and cholera massive infections of the populace may take place from the
contamination of a water supply and the disease be extended over an
entire city. One of the most striking instances of this mode of
extension was in the epidemic of cholera in Hamburg in 1892. There
were two sources of water supply, one of which was infected, and the
cases were distributed in the city in the track of the infected
supply. Many such instances have been seen in typhoid fever. Certain
articles of food, particularly milk, serve as sources of infection.
This is more apt to happen when the organism causing the infection
grows easily outside of the body. A few such organisms entering into
the milk can multiply enormously in a few hours and increase the
amount of infectious material. In all these cases the sick individual
remains a source of infection, for it is almost impossible to avoid
some contamination of the body and the immediate surroundings with the
organisms contained in the discharges.
Transmission by air plays but little part in the extension of
infection. In such a disease as smallpox, where the localization is on
the surface of the body, the organisms are contained in or on the thin
epithelial scales which are constantly given off. These are light, and
may remain floating in the air and carried by air currents just as is
the pollen of plants. There seem to have been cases of smallpox where
other modes of more direct transmission could be excluded and in which
the organisms were carried in the air over a considerable space. All
sorts of intermediate objects, both living and inanimate, such as
persons, domestic animals, toys, books, money, etc., can serve as
conveyors of infection.
Insects play a most important part in the transmission of disease, and
in certain cases, as when a disease is localized in the blood, this is
the only means of transmission. There are three ways in which the
insect plays the rôle of conveyor.
1. The insect may play a purely passive part in that its exterior
surface becomes contaminated with the discharges of the sick person,
and in this way the organisms of disease may be conveyed to articles
of food, etc. The ordinary house fly conveys in this way the organisms
of typhoid and dysentery. Flies seek the discharges not only for food,
but for the purpose of depositing their eggs, and the hairy and
irregular surface of their feet facilitates contamination and
conveyance. When flies eat such discharges the organisms may pass
through the alimentary canal unchanged and be deposited with their
feces; they also often vomit or regurgitate food, and in this way also
contaminate objects. Flies very greedily devour the sputum of
tuberculous patients, and the tubercle bacilli contained in this pass
through them unchanged and are deposited in their feces.
[Illustration: FIG. 19.--TRYPANOSOMES FROM BIRDS. All the trypanosomes
are very much alike. They contain a nucleus represented by the dark
area in the centre and a fur-like membrane terminating in a long
whip-like flagellum. They have the power of very active motion within
the blood.]
2. Diseases which are localized in the blood are transmitted by biting
flies. The biting apparatus becomes contaminated with the organisms
contained in the blood, and these are directly inoculated into the
blood of the next victim. The trypanosome diseases form the best
example of this mode of transmission. The trypanosomes are widely
distributed, exclusively parasitic, flagellated protozoa which live in
the blood of a large number of animals and birds (Fig. 19). They may
give rise to fatal diseases, but in most cases there is mutual
adaptation of host and parasite and they seem to do no harm. One of
the most dangerous diseases in man, the African sleeping sickness, is
caused by a trypanosome, and the disease of domestic cattle in Africa,
nagana, or tsetse fly disease, is also so produced. In certain regions
of Africa where a biting fly, the _Glossina morsitans_, occurs in
large numbers, it has long been known that cattle bitten by these
flies sickened and died, and this prevented the settling and use of
the land. In the blood of the sick cattle swarms of trypanosomes are
found. The source from which the tsetse fly obtained the trypanosomes
which it conveyed to the cattle was unknown until it was discovered
that similar trypanosomes exist in the blood of the wild animals which
inhabit the region, but these have acquired by long residence in the
region immunity or adaptation to the parasite and no disease is
produced. With the gradual extension of settlement of the country and
the accompanying destruction of wild life the disease is diminishing.
Some of the inter-relations of infections are interesting. The
destruction of wild animals in South Africa which, by removing the
sources of nagana, rendered the settlement of the country possible was
due chiefly to the introduction of another infectious disease,
rinderpest, which not only destroyed the wild animals but produced
great destruction of the domestic cattle as well.
The _sleeping sickness_ has many features of interest. In the old
slavery days it was found that the negroes from the Congo region in
the course of the voyage or after they were landed sometimes were
affected with a peculiar disease. They were lethargic, took little
notice of their surroundings, slept easily and finally passed into a
condition of somnolence in which they took no food and gradually died.
There was no extension of the disease and it was attributed to extreme
homesickness and depression. A similar disease has been known for more
than one hundred years on the west coast of Africa, and attracted a
good deal of interest and curiosity on account of the peculiar
lethargy which it produced and from which it has received the name of
"sleeping sickness." Although apparently infectious in its native
haunts, it lost the power of spreading from man upon removal to
regions where it did not prevail. At first confined to a very small
region on the Niger river, it gradually extended with the development
of trade routes and the general increase of communications which trade
brings, until it prevails in the entire Congo basin, in the British
and German possessions in East Africa, and is extending north and
south of these regions. The cause of the disease and its mode of
conveyance was discovered in 1903. The fly _Glossina palpalis_
which conveys the disease is a biting fly about the size of the common
house fly and lives chiefly in the vicinity of water. When such a fly
bites an individual who has sleeping sickness its bite can convey the
disease to monkeys, on whom the transmission experiments were made.
After biting the fly is infectious for a period of two days. After
this it is harmless, unless it again obtains a supply of living
trypanosomes. There is quite a period in which there are no symptoms
of the disease, although trypanosomes are found in the blood and in
the lymph nodes, and the individual is a source of infection. The
peculiar lethargy which has given the disease its name does not appear
until the nervous system is invaded by the parasites. It is impossible
to compute accurately the numbers of deaths from this disease--in the
region of Victoria Nyanza alone the estimates extend to hundreds of
thousands.
3. In the third mode of insect conveyance the insect does not play a
merely passive rôle, but becomes a part of the disease, itself
undergoing infection, and a period in the life cycle of the organism
takes place within it. In all these cases quite a period of time must
elapse before the insect is capable of transmitting the disease; in
malaria, which is the best type of such a disease, this period is ten
days. Malaria is due to a small protozoan, the _Plasmodium
malariæ_, which was discovered by Lavaran, a French investigator,
in 1882. The organism lives within or on the surface of the red blood
corpuscles. It first appears as a very minute colorless body with
active amoeboid movements, and increases in size, attacks a succession
of corpuscles, and finally attains a size as large as or larger than a
corpuscle. The corpuscles attacked become pale by the destruction of
hæmoglobin, swell up and disintegrate, the hæmoglobin becoming
converted into granules of black pigment inside the parasite. Having
attained a definite size the organism forms a rosette and divides into
a number of forms similar to the smallest seen inside the corpuscles;
these small forms enter other corpuscles and the cycle again begins.
This cycle of development takes place in forty-eight hours, and
segmentation is always accompanied by a paroxysm of the disease shown
in a chill followed by fever and sweating which is due to the effect
of substances liberated by the organism at the time of segmentation. A
patient may have two crops of the parasite developing independently in
the blood, and the two periods of segmentation give a paroxysm for
each, so that the paroxysms may appear at intervals of twenty-four
hours instead of forty-eight (Fig. 20). This cycle of development may
continue for an indefinite time, and there may be such a rapid
increase in the parasites as to bring about the death of the
individual; but with him the parasite would also perish, for there
would be no way of extending the infection and providing a new crop.
The disease has been transmitted by injecting the infected blood into
a normal individual.
[Illustration: FIG. 20.--PART OF THE CYCLE OF DEVELOPMENT OF THE
ORGANISM OF MALARIA, _a-g_, Cycle of forty-eight hour development, the
period of chill coinciding with the appearance of _f_ and _g_ in the
blood. The organisms _g_, which result from segmentation, attack other
corpuscles and a new cycle begins. _h_, The male form or
microgametocyte, with the protruding and actively moving spermatozoa,
one of which is shown free. _i_ and _j_ are the macrogametes or female
forms. _k_ shows one of these in the act of being fertilized by the
entering spermatozoön. The differentiation into male and female forms
takes place in the blood, the further development of the sexual cycle
within the mosquito.]
If a mosquito of the species _anopheles_ bites the affected
person, it obtains a large amount of blood which contains many
parasites. Within the mosquito the parasite undergoes a further
development into male and female sexual forms, which may also form in
the blood, termed respectively _microgametocyte_ and _macrogamete_.
From the microgametocyte small flagellate bodies, the male sexual
elements _microgametes_ or _spermatozoa_, develop and fertilize the
_macrogametes_; after fertilization this develops into a large body,
the _oöcyst_ which is attached to the wall of the stomach of the
mosquito. Within the oöcyst, innumerable small bodies, the
sporozoites, develop, make their way into the salivary glands and are
injected into the individual who becomes the prey of the mosquito,
and again the cycle of development begins. The presence of the
parasite within the mosquito does not constitute a disease. So far as
can be determined, life goes on in the usual way, and its duration in
the insect is not shortened.
The nature of the parasite which produces yellow fever is unknown, for
it belongs to the filterable viruses; the infectious material,
however, has been shown by inoculation to exist in the blood, and the
disease is transmitted by a mosquito of another species, the
_stegomyia_. The development cycle within this takes a period of
twelve days, which time must elapse after the mosquito has bitten
before it can transmit the disease. Here again the mutual
interdependence of knowledge is shown. Nothing could have seemed less
useful than the study of mosquitoes, the differentiation of the
different species, their mode of life, etc., and yet without this
knowledge discoveries so beneficial and of such far-reaching
importance to the whole human race as that of the cause and mode of
transmission of malaria and yellow fever would have been impossible;
for it could easily have been shown that the ordinary _culex_
mosquito played no rôle. The rôle which insects may play in the
transmission of disease was first shown by Theobald Smith in this
country, in the transmission by a tick of the disease of cattle known
as Texas fever. The infecting organism _pyrosoma bigenimum_ is a
tiny pear-shaped parasite of the red corpuscles. Smith's
investigations on the disease, published in 1893, is one of the
classics in medicine, and one of the few examples of an investigation
which has not been changed or added to by further work.
One of the most interesting methods of extension of infection, showing
on what small circumstances infection may depend, is seen in the case
of the hookworm disease, which causes such devastation in the Southern
States. The organism which produces the disease, the _Uncinaria_,
belongs to the more highly developed parasites, and is a small round
worm one-third of an inch long. The worms which inhabit the intestines
have a sharp biting mouth by which they fasten themselves to the
mucous membrane and devour the blood. The most prominent symptom of
the disease is anæmia, or loss of blood, due not only to the direct
eating of the parasite, but to bleeding from the small wounds caused
by its bite. Large numbers of eggs are produced by the parasite which
are passed out with the feces, which becomes the only infectious
material. In a city provided with water-closets and a system of
sewerage there would be no means of extension of infection. The eggs
in the feces in conditions of warmth and moisture develop into small
crawling larvæ which can penetrate the skin, producing inflammation of
this, known in the region as the ground itch. The larvæ enter the
circulation and are carried to the lungs, where they perforate the
capillaries and reach the inner surface; from this they pass along the
windpipe, and then by way of the gullet and stomach reach their
habitat, the small intestine. Unfortunately, the habits and poverty of
the people in every way facilitate the extension of the infection.
There is no proper disposal of the feces, few of the houses have even
a privy attached to them, and the feces are distributed in the
vicinity of the houses. This leads to contamination of the soil over
wide areas. Most of the inhabitants of the country go barefoot the
greater part of the year, and this gives ready means of contact with
the larvæ which crawl over the surface of the ground. The disease is
necessarily associated with poverty and ignorance, the amount of blood
is reduced to a low point, and industry, energy and ambition fall with
the blood reduction; the schools are few and inefficient; the children
are backward, for no child can learn whose brain cells receive but a
small proportion of the necessary oxygen; and a general condition of
apathy and hopelessness prevails in the effected communities. The
control of the disease depends upon the disinfection of the feces, or
at least their disposal in some hygienic method, the wearing of shoes,
and the better education of the people, all of which conditions seem
almost hopeless of attainment. The infection is also extended by means
of the negroes who harbor the parasite, but who have acquired a high
degree of immunity to its effects and whose hygienic habits are even
worse than those of the whites. The organism was probably imported
with the negroes from Africa and is one of the legacies of slavery.
The diseases of animals are in many ways closely linked with those of
man. In the case of the larger parasites, such as the tapeworms and
the trichina, there is a direct interchange of disease with animals,
certain phases of the life cycle of the organisms are passed in man
and others in various of the domestic animals. A small inconspicuous
tapeworm inhabits the intestine of dogs and seems to produce no ill
effects. The eggs are passed from the dog, taken into man, and result
in the formation of large cystic tumors which not infrequently cause
death. Where the companionship between dog and man is very close, as
in Iceland, the cases are numerous.
Most of the diseases in animals caused by bacteria and protozoa are
not transmitted to man, but there is a conspicuous exception. Plague
is now recognized as essentially an animal disease affecting rats and
other small rodents, and from these the disease from time to time
makes excursions to the human family with dire results. The greatest
epidemics of which we have any knowledge are of plague. In the time of
Justinian, 542 B.C., a great epidemic of plague extended over what was
then regarded as the inhabited earth. This pandemic lasted for fifty
years, the disease disappeared and appeared again in many places and
caused frightful destruction of life. Cities were depopulated, the
land in many places reverted to a wilderness, and the works of man
disappeared. The actual mortality cannot be known, but has been
estimated at fifty millions. Plague played a large part in the
epidemics of the Middle Ages. An epidemic started in 1346 and had as
great an extension as the Justinian plague, destroying a fourth of the
inhabitants of the places attacked; and during the fifteenth and
sixteenth and seventeenth centuries the disease repeatedly raised its
head, producing smaller and greater epidemics, the best known of
which, from the wonderful description of De Foe, is that of London in
1665, and called the Black Death. Little was heard of the disease in
the nineteenth century, although its existence in Asia was known. In
1894 it appeared in Hong Kong, extended to Canton, thence to India,
Japan, San Francisco, Mexico, and, in fact, few parts of the tropics
or temperate regions of the earth have been free from it. Mortality
has varied greatly, being greatest in China and in India; in the last
the estimate since 1900 is seven million five hundred thousand deaths.
The disease is caused by a small bacillus discovered in 1894 which
forms no spores and is easily destroyed by sunlight, but in the dark
is capable of living with undiminished virulence for an indefinite
time. The disease in man appears in two forms, the most common known
as bubonic plague, from the great enlargement of the lymph nodes,
those of the groin being most frequently affected. The more fatal form
is known as pneumonic plague, and in this the lungs are the seat of
the disease.
In the old descriptions of the disease it was frequently mentioned
that large numbers of dead rats were found when it was prevalent, and
the most striking fact of the recent investigations is the
demonstration that the infection in man is due to transference of the
bacillus from infected rats. There are endemic foci of the disease
where it exists in animals, the present epidemic having started from
such a focus in Northern China, in which region the _Tarabagan_,
a small fur-bearing animal of the squirrel species, was infected. Rats
are easily infected, the close social habits of the animal, the vermin
which they harbor, and the habits of devouring their dead fellows
favor the extension of infection. The disease extends from the rat to
man chiefly by means of the fleas which contain the bacilli, and in
cases of pneumonic plague from man to man by means of sputum
infection. The disease once established in animals tends to remain,
the virus being kept alive by transmission from animal to animal, and
the persistence of the infection is favored by mild and chronic cases.