Practical preventive medicine

Front matter

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Foreword

The following pages represent an endeavor to briefly present the salient features of modern preventive medicine. In a desire to cover the entire field, a great condensation has been effected, which in many instances perhaps results in an inadequate consideration of important subjects. It is believed this represents the minimum knowledge of the subject which a student of medicine or a practitioner of medicine should be expected to possess.

While admittedly brief, the student or reader desiring a broader knowledge will be assisted by the numerous references given. These have been particularly designed to introduce the student to the vast stores of public health literature which exist in the publications of various departments of the United States government. Any individual especially interested in this field should make every effort to acquire those listed- and keep in touch with the new publications issued.

The medical profession can play an important role in the field of preventive medicine and public health. At present physicians are neglecting their opportunities.. If this neglect continues the opportunities will lessen and the field will be taken away from physicians by a changing public sentiment. If these pages will bring home to medical students and practitioners a realization of their public health responsibilities and stimulate co-operation with public health authorities, the labor of their preparation will have been abundantly rewarded.

No originality is claimed for the material here presented. All available sources of information have been freely utilized. Special acknowledgement should be made of the utilization of Rosenau's "Preventive Medicine and Hygiene" to which the specialist or advanced student will naturally turn, and to the report of the Committee on Standard Regulations of the American Public Health Association.

Mark F. Boyd.

Galveston, Texas, November, 1920

Chapter I Introduction

Preventive Medicine may be defined as that branch of applied biology which seeks to reduce or eradicate disease by removing or altering the responsible etiological factors. Included within its scope are two subjects which are often confused with it, these are hygiene and sanitation respectively. Hygiene is the proper care of. the body to permit the normal functioning of the various organs and tissues, while sanitation is the proper cleanliness of the environment.

Since preventive medicine requires a complete knowledge of the etiology of disease for its application, it is apparent that deficiencies of etiologic knowledge must necessarily limit the scope of successful work. There are however five groups of diseases whose etiology is sufficiently well known to warrant their classification as preventable. The groups are:

(I) Diseases produced as the result of the invasion of the body by micro-organisms;

(II) Diseases the result of a faulty or deficient diet;

(III) Diseases the result of unhygienic or insanitary conditions of employment;

(IV) Diseases arising as the result of the puerperal state; and (V) Diseases transmitted from parent to offspring. Despite the fact that the number of diseases included in the

above groups is limited, this handicap is very much reduced by the fact that the diseases included in the above groups are for the most part, of considerable importance as causes of morbidity and mortality, so that effective control measures directed against them will accomplish a great deal in reducing the hazards of life. Their importance may be judged from the following mortality statistics from the registration area of the United States:

2 17

TABLE I

1912 1913

Per cent, of total population of U. S. in registration area. .. . 63.2 65.1

Total deaths, all causes 838,251 890,848

Deaths, disease of group 1 287,645 304,580

Deaths, disease of group 2 4,409 15,005

Deaths, disease of group 3 156 171

Deaths, disease of group 4 9,035 10,010

Deaths, disease of group 5 Not a direct cause

of death.

While the diseases of group 5 are not of importance as causes of mortality, they nevertheless exert a certain definite influence which we will consider later.

Directly as well as indirectly these diseases are the cause of immense economic losses. These affect both the individual and the social fabric. The following economic losses may be enumerated:

(a) Temporary or permanent disablement of the patient as an immediate result of the disease, resulting to the individual in temporary or permanent loss of earning, and to society in the loss of productive labor;

(b) Sequela? or complications which permanently impair the individuals usefulness or hasten death from other causes;

(c) Expenses of illness, which if due to preventable illness, must be regarded as an economic loss;

(d) The expense incident to the establishment and maintenance of public hospitals, asylums and charitable agencies to relieve want arising from disabling sickness;

(e) Making vast regions of the earth's surface uninhabitable for the civilized races; while

(/") Preventable diseases of the domesticated food producing animals may cause such inroads into their numbers that animal foods become scarce and consequently high in price. This is due both to a destruction of the animals as well as to a decreased productiveness of those surviving.

With most of the diseases included in the foregoing groups our available knowledge is sufficiently adequate to justify us in classifying them as preventable. Their continued presence with us is chiefly due to the lack of ways and means for placing effective control measures in operation. It may never be practicable to place in operation in civil life the drastic, but nevertheless effective measures which have made the military application of preventive medicine so brilliant, though the experience with military discipline emphasizes the administrative

INTRODUCTION 1 9

difficulties which are encountered in the civil application of these measures, where tact rather than force must win the point. Conceiving a population wherein an adequately organized defensive and offensive body was available to apply proper measures, we might expect that their continued application would have certain effects. Among these we may prophesy the following:

(a) An increase in the period of expectation of life, that is ' the probable duration of the life of the hypothetical average individual. This change will be accompanied by the following phenomena: (i) There will first be a gradual diminution in the total death rate, due to the gradual disappearance of the preventable diseases as causes of mortality; (2) A change in the age distribution of the deaths will next be apparent. The majority of the conditions we are considering are most active as causes of death among the ages below thirty. Their elimination will of course permit a larger proportion of individuals to survive to ages beyond thirty, and as the change takes place the number of deaths above thirty will slowly increase, proportionate to the decrease in the deaths below thirty. (3) The conditions which are operative as causes of death at the age periods beyond thirty, other than those of our particular groups will gradually be found to be responsible for an increasingly greater number of deaths. This state of affairs is not necessarily alarming, but rather is encouraging, as indicating that a larger proportion of individuals are permitted to survive to middle life and older periods. This is, in our opinion, the most rational explanation of the increases which have been observed in the mortality from carcinoma and cardio-renal disease. The completion of this cycle of transformation will probably leave us with a crude death rate from all causes very nearly the same as before the days of even the most feeble preventive work. The important difference will be in the age distribution of the deaths. Instead of the many dying young, the majority will survive to middle life or old age. The individual will have a better chance of living what may be said to be a life of "normal" duration.

(b) Unhealthful regions of the earth will be made habitable, consequently human overcrowding will be relieved, and more of the earth's treasures will be available for mankind.

(c) The economic productiveness of the individual as well as of the race will be increased. As a consequence individual and national wealth will increase and poverty and want diminish.

(d) The supply of animal foods will increase and only be limited b y the available roughage.

(e) Lastly we may prophesy an improvement in the general physical condition of the race.

The effective operation of the necessary machinery to apply the etiological knowledge we shall briefly sketch may be said to be the problem. It is one of great difficulty and complexity and its complete solution is far distant. Partial means to control the diseases of group one are as we shall presently see, of considerable antiquity, and arose from a recognition that persons infected with communicable diseases were a danger to the public. Thus there developed the field of public health. Originally it arose from a purely selfish attitude on the part of society as a whole to protect itself from certain infected individuals, whose objectionable characteristics arose from no fault of their own. To-day we find a changing attitude, a realization that if society requires protection by enforcing certain restrictive measures on individuals innocent of crime, justice demands that these persons receive consideration. In addition the field of public health has recently come to have a broader scope, due to the realization that many of our problems, if not all, have a sociological foundation, and that a divorce is not always possible. Relief will only be secured when these associated problems are solved.

In a general way our problem of disease prevention has two aspects, namely curative and prophylactic.

The curative aspect is primarily the problem of the practicing physician and when effectively solved will be manifested by a lowered case-mortality. The physician will be assisted in its solution by the development of methods and the provision of facilities for the making of prompt diagnoses and the development and application of specific therapeutic measures. This aspect is clearly a problem of the practicing physician in his relation to the individual requiring his services.

On the other hand the prophylactic aspect can only give satisfactory results when an entire social unit, such as any community, takes cognizance of its problem and attacks it with all the resources at its collective command. For this purpose our social and political units have delegated power and authority to certain officials for the protection of the public health. The problem confronting the officials relates particularly to a reduction in the number of cases of preventable diseases in the population under their care. They are not professionally interested in disease from the individualistic standpoint of the physician. The degree of success achieved by

INTRODUCTION 21

these officials will be directly in proportion to the degree in which they educate their public in the principles they are trying to apply. Without the co-operation of the medical profession and the laity health authorities will accomplish very little effective work.

In general the field of public health work may be said to have the following scope:

(a) Improved personal hygiene of all individuals, including better standards of personal cleanliness, better dietaries, reasonable working hours, recreation and adequate clothing.

(b) Improved standards of domestic and public sanitation, including relief from overcrowding, proper illumination, heating and ventilation, water supply, excreta disposal, etc.

(c) Improved sanitation of places of employment.

id) The immunization of susceptible persons and the control of infected persons.

(e) The improvement of the breeding stock of the human race by the elimination of the physically and mentally unfit from reproduction.

(/) The provision of facilities for aiding physicians in the diagnosis and care of their patients, i.e., laboratories, hospitals and clinics.

Landmarks in the Development of Preventive Medicine. — The only practices of the ancients which we at this day may consider to be preventive measures based upon a firm, rational foundation as we understand the subject, are found described in the Mosaic law. All other practices of the ancients designed to prevent diseases are clearly allied with religion and superstition, and hence were of little importance and no value. Therefore the Mosaic instructions when interpreted in the light of present day knowledge have an immense importance. These practices however do not seem to have been copied by contemporaneous Gentile races.

Aside from the foregoing the earliest public health practice which has survived to the present day is maritime quarantine, which was developed by the mediaeval Italian cities of Venice and Genoa when at the height of their commercial splendor, as a protection against the introduction of plague from oriental ports. At this time the ideas of disease transmission were very vague, but a suspicion of the transmissibility of some seems to have existed. A little later, in 1546, Geronimo Fracastorius published in Venice a work entitled "De contagionibus et contagiosis morbis et curatione" in which was first definitely advanced the doctrine of contagion. He divided infections into three classes: (i) Those infecting by immediate contact, (2) Those infecting through intermediate agents, such as fomites and (3) Those infecting at a distance or through the air In 1659 Kircher, and in 1675 van Leeuwenhoek first observed and described living organisms too small to be seen by the naked eye. Kircher in 1671 suggested that various infections were the result of the activity of these minute organisms. Kircher's views were received with scepticism by his contemporaries, and later, in 1762, Plenciz of Vienna again advanced the same views. These theories however did not gain headway until the following century, when they were demonstrated scientifically.

The first attempt at artificial active immunization among European nations must be credited to Lady Mary Wortley Montague, who from 1717 to 1721 introduced into England from Constantinople, the process of variolation as a protection against smallpox. This was an event whose importance has been overshadowed by the employment of an attenuated virus for the same purpose by Jenner. His discovery was first published in 1798.

In 1843, Oliver Wendell Holmes, an American physician and author of note, first called attention to the contagiousness of puerperal fever. The activity of water as a route for the transfer for infective agents was first recognized in 1854 by Dr. Snow in connection with the famous Broad Street well cholera outbreak. Three years later, Dr. Taylor recognized the similar activity of milk in an outbreak of typhoid at Penrith. The first scientific demonstration of the transmissible character of an infectious disease was performed by Villemin with tuberculosis in 1865, while the first demonstration of the etiological relation of micro-parasites to disease was accomplished by Pasteur in the case of anthrax in 1876, thus substantiating the earlier beliefs of Kircher and Plenciz. Patrick Manson recognized the first known insect transmitted disease, when he found that mosquitoes transmit Filaria bancrofti.

From the time of Jenner no progress in artificial immunization was made until Pasteur demonstrated the protective power of his anthrax vaccine on sheep in 1881, and in 1885 extended the same principle to the treatment of rabies.

In 1893, Smith and Kilbourne, two Americans, discovered the cause and means of transmission of the first known insect transmitted protozoal disease, namely Texas fever of cattle.

The importance of carriers in the perpetuation of typhoid

INTRODUCTION fever was first recognized by Robert Koch, who called attention to them in 1902. Carriers of the diphtheria bacillus had been observed before this, but their importance was not recognized. The results which can accompany the application of the principles of preventive medicine received their first great popular demonstration by Gorgas, when he eradicated yellow fever and malaria from Havana and the Canal Zone. This accomplishment may be considered to mark the beginning of active public interest in the possibilities of preventive medicine, a situation which may be said to characterize the present day.

Sedgwick: Principles of Sanitary Science and the Public Health. Chapters II, III, IV.

Leviticus — Chapter XIII: 14 Deuteronomy — Chapter XXIII : 1 3 Eager: The Early History of Quarantine.

Yellow Fever Institute Bull. No. 12, U.S.P.H. and M.H. Service. Mortality Statistics. Published annually by the Bureau of Census since 1900.

Chapter II Sources Of Infection

i. As a result of the invasion of a population by a given species of pathogenic micro-organisms certain phenomena are produced, some of which are of present interest, particularly those dealing with the incidence or prevalence of disease. This constitutes the field of epidemiology, which may be defined as the science treating of the sources and routes of infection and their activities, as manifested by the incidence or prevalence of an infectious disease in a given population.

2. Before proceeding further certain fundamental definitions had best be presented:

(a) An infectious disease is produced as the result of the invasion of the tissues of an animal or plant by a living organism. Because the invading organism may be in many instances transferred from individual to individual of the host species, these diseases are perhaps more aptly termed communicable. Those communicable diseases which are transmitted exclusively or nearly so, directly from host to host are spoken of as contagious, or popularly as "catching."

(b) Following the invasion of the host by an organism capable of producing disease, a variable period elapses before the characteristic symptoms are manifested and during which the ordinary health is apparently maintained. This period is the incubation period. With some infections, as small pox and measles, its length is very constant, with others such as typhoid, it is subject to considerable variation. Its duration is probably dependent upon the number and virulence of the invading organisms on the one hand, and to the specific degree of the host's resistance on the other.

(c) A primary case of a communicable disease is the first case arising within a given area, which is the ascribed source

SOURCES OF INFECTION 25

of the infective agents transferred to other individuals who later develop the same infection.

A secondary case of a communicable disease arises as a result of infection received from a pre-existing known case, with which it has been associated. Both are relative terms.

A vector is any intermediate object which serves to transfer infective agents from one host to another. The use of this term is commonly restricted to those insects which occupy this position.

An epidemic is the occurrence within a limited time among a limited population, of an unusual number of cases of communicable disease.

A pandemic is similar to an epidemic, except the population concerned is much larger, that of an entire continent or the world.

Endemic refers to the usual prevalence of a communicable disease among a limited population.

Epizootic is a term analagous to epidemic, referring to the diseases of lower animals.

3. Sources of Infection. — As a result of extensive bacteriologic researches as well as epidemiologic studies of similar magnitude, performed by scientific observers everywhere it may be stated as axiomatic that:

All communicable diseases arise from the invasion of a susceptible host by organisms derived from a pre-existing invaded host. The pre-existing invaded host is known as the source of infection.

It has long been observed as a fairly constant characteristic that a greater incidence of these diseases was found among individuals in contact with those sick with the same disease, although many cases certainly occur that do not present such a history. At one time in the early days of bacteriologic science it was believed that patients had received infection from organisms that had developed in, and were received from a patient's environment. In other words, the invaders were saprophytes which incidentally possessed pathogenic powers. Such is apparently the actual case with some of the pathogenic fungi. On the other hand, the pathogenic bacteria are apparently of a highly parasitic nature. Bacteriological researches have failed to demonstrate that they can live a sustained vegetative extracorporeal existence. When found without the body it is under circumstances which indicate they are existing within a definite radius surrounding an infected person, and their origin has been undeniably the excretions and secretions of the infected person, and it has been further found that these micro-organisms, with certain few notable exceptions, are only capable of a rather transitory extracorporeal existence, during which period their vital activities are more or less suspended and their vitality tends to decrease.

4. The sources of infection as defined above may be classified as follows:

(a) Typical Cases. — These are individuals in whom the disease presents the clinical manifestations typical of the disease.

(b) Atypical Cases (Abortive or Missed). — These are individuals in whom the clinical symptoms are indefinite or indistinctive, and as a consequence are difficult to diagnose. Furthermore the illness is usually mild. Bacteriologic assistance is frequently required for a diagnosis, though sometimes, when occurring as secondary cases where the primary case is known, they may be diagnosed clinically. Their proportion to the typical cases varies with different diseases.

(c) Carriers are apparently healthy persons who are harboring the specific organisms of communicable diseases and discharging them from their bodies. They are encountered in well defined types.

1. Incubatory carriers, i.e., individuals who are actually in the incubation period of a given infectious disease and will shortly manifest the characteristic symptoms.

2. Healthy carriers, i.e., individuals who throughout the entire period of their harborage never present clinical manifestations that may be referable to their parasitism.

3. Convalescent carriers, i.e., individuals who have recovered from an attack of the disease in question, but who continue to harbor the specific infective agents. According to the duration of their infectivity, convalescent carriers are commonly arbitrarily divided into two groups:

(a) Temporary carriers, whose infectivity subsides in three months following the development of their convalescence, and

(b) Chronic carriers, in whom the infective agents persist for periods longer than three months.

It must be remembered that the healthy and convalescent carriers possess an active resistance to the micro-organisms they are harboring, so that while they themselves are protected they endanger their associates.

The proportion of carriers to cases at any given time with a given disease is subject to wide fluctuations and is difficult of

SOURCES OF INFECTION 27

accurate estimation. The carrier condition, in so far as it relates to the actual discharge of micro-organism is intermittent. The particular secretions or excretions are not continuously infective, even though the micro-organisms are continually present in the body. In this respect the carrier condition is closely related to the group of sources known as latent cases.

(d) Latent Cases. — With certain infections the active clinical manifestations tend to spontaneously subside, though the micro-organisms themselves persist, apparently in a latent condition, for long and variable periods of time, later to renew their activity when from some intercurrent cause their host's resistance is reduced. During the period of their latency they are not discharged from the body. Latent infection, for example, is observed in syphilis, tuberculosis and glanders.

(e) Lower Animals as Sources of Infection. — The foregoing observations relate particularly to the so-called diseases of man, for which human beings serve solely as sources of dissemination. On the other hand, a certain group of human infections, such as rabies, glanders, anthrax, etc., are not due to agents derived from infected human beings, but from infected animals, though the role for the animal as a source of infection is closely parallel.

5. Examples of disease classification in the foregoing types:

(a) Atypical forms are encountered in the following infections :

Typhoid fever Poliomyelitis

Septic sore throat Typhus Fever

Measles Diphtheria

Asiatic cholera Small pox

Meningococcus meningitis Yellow fever Scarlet fever

(b) Incubatory carriers have been observed in the following:

Typhoid Fever Whooping cough

Paratyphoid Fever Measles

Malta Fever Small pox

Dysentery (amoebic) Anthrax Diphtheria

(c) Healthy carriers of micro-organisms producing the following diseases have been observed:

Typhoid Fever Pneumonia

Paratyphoid Fever Meningococcus meningitis

Dysentery (amoebic and Gonorrhoea bacillary) Plague

Cholera Poliomyelitis

Diphtheria Malaria

Scarlet Fever

(d) Convalescent carriers of the following:

Typhoid Fever Dysentery (bacillary and

Paratyphoid Fever amoebic)

Diphtheria Cholera

Pneumonia Meningococcus meningitis

Poliomyelitis African Sleeping Sickness

6. Exit of Micro-organisms from the Body of Infected Persons.

Different species of micro-crganisms have adapted themselves to different pathways of exit from the body of infected persons. In order that the propagation of the species be maintained, it is of course necessary that colonizing individuals be disseminated. For the most part the adaptation is to the natural avenues of exit used for the physiological discharge of surplus secretions or the excretions of the body. To a considerable extent the adaptation is specific, i.e., certain of these pathways are more certain to contain micro-organisms producing a given disease, than others. For example the following discharges are the means of exit of the infective agents producing:

(a) Feces: Typhoid, dysentery, cholera, etc.

(b) Urine: Typhoid, etc.

(c) Sputum: Tuberculosis, pneumonia, etc.

(d) Saliva: Rabies.

(e) Nasopharyngeal -secretions: Diphtheria, cerebro-spinal meningitis, etc.

(/") Sweat: Typhoid.

(g) Milk: Typhoid, Malta fever, tuberculosis.

Qi) Epithelial desquamations: Small pox.

{i) Purulent discharges: Scarlet Fever.

(j) Blood : Malaria, African sleeping sickness.

(k) Lachrymal secretion: Conjunctivitis.

(/) Placental circulation: Syphilis, tuberculosis.

Chapin: Sources and Modes of Infection. Chapter II.

Ledingham and Arkwright: The Carrier Problem in Infectious Disease.

Simon: Human Infection Carriers.

Rosenau: Preventive Medicine and Hygiene. Chapter VII, pp. 362.

Chapter III The Dissemination Of Infective Agents

i. Stage of Disease in Relation to Dissemination. — (a)

Stage of Invasion and Incipiency. — With some diseases such as tuberculosis, when the micro-organisms are in deepseated lesions not in contact with passage ways to the exterior, the infected person is of little or no active danger to others as a source of infection. On the other hand, with diseases such as diphtheria, where the micro-organisms are in superficial situations having ready access to the exterior, this period is of great importance.

(b) Non-infective Stages of Active Disease. — When syphilitic infection progresses to the tertiary and para-syphilitic stages, these as long as they persist are non-infectious. On the other hand, they partake of the character of latent infections, inasmuch as relapses to lesions of the secondary type may occur.

2 . Portals Through Which Infective Agents Enter the Body. — Different infective agents have adapted themselves to varying portals of entrance into the body and for the most part, if introduced by a route other than the one to which adapted, fail to gain a foothold. The principal body orifices again play an important part, particularly the orifices of entrance, rather than those of exit. After having passed the threshold of this portal, the infective agents may proceed by divergent pathways to penetrate the physiological interior of the body.

The mouth and the nose are portals of entrance of greatest importance from the standpoint of the number of infective agents which are introduced through them.

After passing this threshold, some, as the typhoid bacillus may go onward to the intestinal tract, others as the pneumococcus procede to the respiratory passages, while others as the diphtheria bacillus, will remain near the point of their entry in the naso-pharynx.

Areas of specialized epithelium, as for example that of the conjunctiva or genitalia, are particularly well suited to some infective agents.

3°

Others, by various means, usually due to some form of traumatism, (inoculation) directly penetrate the outer epithelium of the body to the subcutaneous tissues, and thence gain access to the circulation or to the central nervous system.

3. Dosage. — The mere introduction of a given infective agent into the body will not necessarily mean that the micro-organisms will succeed in gaining a foot-hold. The success of a given transfer will vary directly with the number and virulence of the organisms transferred, and indirectly with the resistance of the individual invaded. These factors also influence the duration of the period of incubation.

4. The Extracorporeal Existence of Infective Agents. — The infective agents are for the most part highly specialized parasites that require for their existence the conditions of temperature, humidity, moisture, and nutrition that they encounter within the body of their host. When these conditions are not available their vegetative and reproductive activities become sluggish, and if prolonged the vitality of the organism may become impaired. Furthermore the colonizing individuals when expelled from the body of their host are ordinarily immediately exposed to the action of the natural germicidal agencies, sunlight and desiccation. (See Table II.) In consequence of these factors they become rapidly attenuated and are soon

TABLE II

Resistance to Sunlight and Desiccation of certain non-spore forming Bacteria.

(Compiled from various authorities)

Organism

Sunlight direct

Material

Authority

Sp. pallida

Diffuse — II>2 nrs-

Cultures

Zinsser

Gonococcus

Pneumococcus

i hr.

Meningococcus

Less than 24 hrs.

B. mallei

24 hrs.

Cholera vibrio

B. pestis

4-5 hrs. 6 min.

film culture

B. diphtheria;

B. typhosus fabric

Sternberg

2 min.

fabric

B. tuberculosis

2 min.

fabric

45 nrs.

sputum

Mitchell & Crouch

M. melitensis

THE DISSEMINATION OP INFECTIVE AGENTS

Organism

Desiccation

Material

Authority

Xotes

Sp. pallida

1 hr.

Films

Zinsser

Gonococcus

Pus on fabrics

Pneumococcus

1-4 mo. no

•

Hiss & Zinsser; In fine spray spu-

light

turn, 3*2 hrs., ordinary light and

temperature-Wood.

Meningococ-

3 hrs— 2o°C.

Besson

cus

B. mallei

48 hrs.-room temp.

Pus, thin layer

Besson

Cholera vibrio

1 min.

Film

Strong

Dry atmosphere, 6 min., spray wet atmosphere 30 min. in spray. Strong.

B. pestis

5 min.

Film

Strong

Floors — 24-48 hrs.

Adv. Comm. India.

B. diphtherise

5 days .

Film

Desiccated in membrane— 4 mos. still virulent

Park & Williams.

B. typhosus

5 days

Film

B. tuberculosis

8 days

Film

2 mos.

Sputum

M. melitensis

Dust

Med. Fever Com.

7-37 days

Fabric & Urine

Med. Fever Com.

Note: Results refer to action of the full physical force concerned.

destroyed. It is exceedingly rare for them to gain introduction into any medium favorable for multiplication, such as milk. The fortunate individuals who do succeed in reaching a new host are but a small fraction of those which left the source. These circumstances are our salvation.

It must of course be remembered that the spore-forming bacteria such as the anthrax and tetanus bacilli, are exceptions to the rule in so far as survival is concerned, but on the other hand, their numbers probably do not increase during this period.