Preventable Diseases Part 11

When once the outer wall has been pierced, the sack of the city rapidly proceeds. The bacilli multiply everywhere, but seem for some reason to focalize chiefly in the alimentary ca.n.a.l, and especially the middle part of it, the small intestines. After headache, backache, and loss of appet.i.te comes usually a mild diarrh[oe]a. This diarrh[oe]a is due to an attack of the bacillus or its toxins upon certain clumps of lymphoid tissue in the wall of the small intestine, known as the "patches of Peyer." This produces inflammation, followed by ulceration, which in severe cases may eat through the wall of a blood-vessel, causing profuse hemorrhages, or even perforate the bowel wall and set up a fatal peritonitis. The temperature begins to swing from two to five degrees above the normal level, following the usual daily vibration, and ranging from 100 degrees to 101 degrees in the morning up to 102 degrees to 105 degrees in the afternoon. The face becomes flushed.

There is usually comparatively little pain, and the patient lies in a sort of mild stupor, paying little attention to his surroundings. He is much enfeebled and seldom cares to lift his head from the pillow. A slight rash appears upon the surface of the body, but this is so faint that it would escape attention unless carefully looked for. Little groups of vesicles, containing clear fluid, appear upon the chest and abdomen. If one of these faint rose-colored spots be p.r.i.c.ked with a needle and a drop of blood be drawn, typhoid bacilli will often be found in it, and they will also be present in the clear fluid of the tiny sweat blisters.

This condition will last for from ten days to four weeks, the patient gradually becoming weaker and more apathetic, and the temperature maintaining an afternoon level of 102 to 104 degrees. Then, in the vast majority of cases, a little decline of the temperature will be noticed.

The patient begins to take a slight interest in his surroundings. He will perhaps ask for something to drink, or something to eat, instead of apathetically swallowing what is offered to him. Next day the temperature is a little lower still, and within a week, perhaps, will have returned to the normal level. The patient has lost from twenty to forty pounds, is weak as a kitten, and it may be ten days after the fever has disappeared before he asks to sit up in bed.

Then follows the period of return to health. The patient becomes a walking appet.i.te, and, after weeks of liquid diet, will beg like a spoiled child for cookies or hard apples or pie, or something that he can set his teeth into. But his tissues are still swarming with the bacilli, and any indiscretion, either of diet, exposure, or exertion, at this time, may result in forming a secondary colony, or abscess, somewhere in the lungs, the liver, or the muscles. He must be kept quiet and warm, and abundantly, but judiciously, fed, for at least three weeks after the disappearance of the fever, if he wishes to avoid the thousand and one ambuscades set by the retreating enemy.

Now, what has happened when recovery begins? One would suppose that either the bacilli had poisoned themselves, exhausted the supplies of nourishment in the body of the patient, so that the fever had "burnt itself out," as we used to say, or that the tissues had rallied from the attack and destroyed or thrown out the invaders. But, on the contrary, we find that our convalescent patient, even after he is up and walking about, is still full of the bacilli.

To put it very crudely, what has really happened is that the body has succeeded in forming such antidotes against the poison of the bacilli that, although they may be present in enormous numbers, they can no longer produce any injurious effect. In other words, it has acquired immunity against this particular germ and its toxin. In fact, one of our newest and most reliable tests for the disease consists in a curious "clumping" or paralyzing power over cultures of the _Bacillus typhosus_, shown by a drop of the patient's blood, even as early as the seventh or eighth day of the illness. And, while it is an immensely difficult and complicated subject, we are justified in saying that this immunity is not merely a substance formed in the body, the stock of which will shortly become exhausted, but a faculty acquired by the body-cells, which they will retain, like other results of education, for years, and even for life. When once the body has learned the wrestling trick of throwing and vanquishing a particular germ or bacillus, it no longer has much to dread from that germ. This is why the same individual is seldom attacked the second time by scarlet fever, measles, typhoid, and smallpox.

While, however, the individual may be entirely immune to the germs of a given disease, he may carry them in his body in enormous numbers, and infect others while escaping himself.

This is peculiarly true of typhoid, and we are beginning to extend our sanitary care over recovered patients, not merely to the end of acute illness, but for the period of at least a month after they have apparently recovered. Several most disquieting cases are on record of so-called "typhoid carriers," or individuals who, having recovered from the disease itself, carried and spread the infection wherever they went for months and even years afterward. This, however, is probably a rare state of affairs, though a recent German health bulletin reports the discovery of some twenty cases during the past year. The lair of the bacilli is believed to be the gall-bladder.

As to treatment, it may be broadly stated that all authorities and schools are for once practically agreed:--

First, that we have no known specific drug for the cure of the disease.

Second, that we are content to take a leaf out of nature's book, and follow--so to speak--her instinctive methods: first of all, by putting the patient to bed the moment that a reasonable suspicion of the disease is formed; this conserves his strength, and greatly diminishes the danger of serious complications; cases of "walking typhoid" have among the highest death-rates; second, by meeting the great instinctive symptom of fever patients since the world began, thirst, encouraging the patient to drink large quant.i.ties of water, taking care, of course, that the water is pure and sterile. The days when we kept fever patients wrapped up to their necks in woolen blankets in hot, stuffy rooms, and rigorously limited the amount of water that they drank--in other words, fought against nature in the treatment of disease--have pa.s.sed. A typhoid-fever patient now is not only given all he wants to drink, but encouraged to take more, and some authorities recommend an intake of at least three or four quarts, and, better, six and eight quarts a day.

This internal bath helps not only to allay the temperature, but to make good the enormous loss by perspiration from the fevered skin, and to flush the toxins out of the body.

Third, by liberal and regular feeding chiefly with some liquid or semi-liquid food, of which milk is the commonest form. The old att.i.tude of mind represented by the proverb, "Feed a cold and starve a fever,"

has completely disappeared. One of the fathers of modern medicine asked on his death-bed, thirty years ago, that his epitaph should be, "He fed fevers."

Fourth. We respond to the other great thirst of fever patients, for coolness, by sponge baths and tub baths, whenever the temperature rises above a certain degree.

Simple as these methods sound, they are extremely troublesome to put into execution, and require the greatest skill and judgment in their carrying out. But intelligent persistence in the careful elaboration of these methods of nature has resulted in already cutting the death-rate in two,--from fifteen or twenty per cent to less than ten per cent,--and where the full rigor of the tub bath is carried out it has been brought down to as low as five per cent.

Meanwhile the bacteriologists are steadily at work on a vaccine or ant.i.toxin. Wright, of the English Army Medical Staff, has already secured a serum, which has given remarkable results in protecting regiments sent out to South Africa and other infected regions.

Chantemesse has imported some six hundred successive cases treated with an ant.i.toxin, whose mortality was only about a third of the ordinary hospital rate, and the future is full of promise.

CHAPTER X

DIPHTHERIA

That was a dark and stern saying, "Without the shedding of blood there is no remission," and, like all the words of the oracles, of limited application. But it proves true in some unexpected places outside of the realm of theology. Was there something prophetic in the legend that it was only by the sprinkling of the blood of the Paschal Lamb above the doorway that the plague of the firstborn could be stayed? To-day the guinea-pig is our burnt offering against a plague as deadly as any sent into Egypt.

Scarcely more than a decade ago, as the mother sat by the cradle of her firstborn, musing over his future, one moment fearfully reckoning the gauntlet of risks that his tiny life had to run, and the next building rosy air-castles of his happiness and success, there was one shadow that ever fell black and sinister across his tiny horoscope. Certain risks there were which were almost inevitable,--initiation ceremonies into life, mild expiations to be paid to the G.o.ds of the modern underworld, the diseases of infancy and of childhood. Most of these could be pa.s.sed over with little more than a temporary wrinkle to break her smile. They were so trivial, so comparatively harmless: measles, a mere reddening of the eyelids and peppering of the throat, with a headache and purplish rash, dangerous only if neglected; chicken-pox, a child's-play at disease; scarlatina, a little more serious, but still with the chances of twenty to one in favor of recovery; diphtheria--ah! that drove the smile from her face and the blood from her lips. Not quite so common, not so inevitable as a prospect, but, as a possibility, full of terror, once its poison had pa.s.sed the gates of the body fortress. The fight between the Angel of Life and the Angel of Death was waged on almost equal terms, with none daring to say which would be the victor, and none able to lift a hand with any certainty to aid.

Nor was the doctor in much happier plight. Even when the life at stake was not one of his own loved ones,--though from the deadly contagiousness of the disease it sadly often was (I have known more doctors made childless by diphtheria than by any other disease except tuberculosis),--he faced his cases by the hundred instead of by twos and threes. The feeling of helplessness, the sense of foreboding, with which we faced every case was something appalling. Few of us who have been in practice twenty years or more, or even fifteen, will ever forget the shock of dismay which ran through us whenever a case to which we had been summoned revealed itself to be diphtheria. Of course, there was a fighting chance, and we made the most of it; for in the milder epidemics only ten to twenty per cent of the patients died, and even in the severest a third of them recovered. But what "turned our liver to water"--as the graphic Oriental phrase has it--was the knowledge which, like Banquo's ghost, would not down, that while many cases would recover of themselves, and in many border-line ones our skill would turn the balance in favor of recovery, yet if the disease happened to take a certain sadly familiar, virulent form we could do little more to stay its fatal course than we could to stop an avalanche, and we never knew when a particular epidemic or a particular case would take that turn.

"Black" diphtheria was as deadly as the Black Death of the Middle Ages.

The disease which caused all this terror and havoc is of singular character and history. It is not a modern invention or development, as is sometimes believed, for descriptions are on record of so-called "Egyptian ulcer of the throat" in the earliest centuries of our era; and it would appear to have been recognized by both Hippocrates and Galen.

Epidemics of it also occurred in the Middle Ages; and, coming to more recent times, one of the many enemies which the Pilgrim Fathers had to fight was a series of epidemics of this "black sore throat," of particularly malignant character, in the seventeenth century.

Nevertheless, it does not seem to have become sufficiently common to be distinctly recognized until it was named as a definite disease, and given the t.i.tle which it now bears, by the celebrated French physician, Bretonneau, about eighty years ago. Since then it has become either more widely recognized or steadily more prevalent, and it is the general opinion of pathologists that the disease, up to some thirty or forty years ago, was steadily increasing, both in frequency and in severity.

So that we have not to deal with a disease which, like the other so-called diseases of childhood, has gradually become milder and milder by a sort of racial vaccination, with survival of the less susceptible, but one which is still full of virulence and of possibilities of future danger.

Unlike the other diseases of childhood, also, one attack confers no positive immunity for the future, although it greatly diminishes the probabilities; and, further, while adults do not readily or frequently catch the disease, yet when they do the results are apt to be exceedingly serious. Indeed, we have practically come to the conclusion that one of the main reasons why adults do not develop diphtheria so frequently as children, is that they are not brought into such close and intimate contact with other children, nor are they in the habit of promptly and indiscriminately hugging and kissing every one who happens to attract their transient affection, and they have outgrown that cheerful spirit of comradeship which leads to the sharing of candy in alternate sucks, and the pa.s.sing on of slate-pencils, chewing-gum, and other _objets d'art_ from hand to hand, and from mouth to mouth.

Statistics show that of nurses employed in diphtheria wards, before the cause or the exact method of contagion was clearly understood, nearly thirty per cent developed the disease; and even with every modern precaution there are few diseases which doctors more frequently catch from their patients than diphtheria. It is a significant fact that the risk of developing diphtheria is greatest precisely at the ages when there is not the slightest scruple about putting everything that may be picked up into the mouth,--namely, from the second to the fifth year,--and diminishes steadily as habits of cleanliness and caution in this regard are developed, even though no immunity may have been gained by a mild or slight attack of the disease. The tendency to discourage and forbid the indiscriminate kissing of children, and the crusade against the uses of the mouth as a pencil-holder, pincushion, and general receptacle for odds and ends, would be thoroughly justified by the risks from diphtheria alone, to say nothing of tuberculosis and other infections.

In addition to being almost the only common disease of childhood which is not mild and becoming milder, diphtheria is unique in another respect, and that is its point of attack. Just as tuberculosis seizes its victims by the lungs, and typhoid fever by the bowels, diphtheria--like the weasel--grips at the throat. Its bacilli, entering through the mouth and gaining a foothold first upon the tonsils, the palate, or back of the throat (pharynx), multiply and spread until they swarm down into the larynx and windpipe, where their millions, swarming in the mesh of fibrin poured out by the outraged blood-vessels, grow into the deadly false membrane which fills the air-tube and slowly strangles its victim to death.

The horrors of a death like that can never fade from the memory of one who has once seen it, and will outweigh the lives of a thousand guinea-pigs. No wonder there was such a widespread and peculiar horror of the disease, as of some ghostly thug or strangler.

But not all of the dread of diphtheria went under its own name. Most of us can still remember when the commonest occupant of the nursery shelf was the bottle of ipecac or soothing-syrup as a specific against croup.

The thing that most often kept the mother or nurse of young children awake and listening through the night-watches was the sound of a cough, and the anxious waiting to hear whether the next explosion had a "croupy" or bra.s.sy sound. It was, of course, early recognized that there were two kinds of croup, the so-called "spasmodic" and the "membranous,"

the former comparatively common and correspondingly harmless, the latter one of the deadliest of known diseases. The fear that made the mother's heart leap into her mouth as she heard the ringing croup-cough was lest it might be membranous, or, if spasmodic, might turn into the deadly form later. To-day most young mothers hardly know the name of wine of ipecac or alum, and the coughs of young children awaken little more terror than a similar sound in an adult. Croup has almost ceased to be one of the bogies of the nursery. And why? Because membranous croup has been discovered to be diphtheria, and children will not develop diphtheria unless they have been exposed to the contagion, while, if they should be, we have a remedy against it.

He was a bold man who first ventured to announce this, and for years the battle raged hotly. It was early admitted that certain cases of so-called membranous croup in children occurred after or while other members of the family or household had diphtheria; and for a time the opposing camps used such words as "sporadic" or scattered croup, which was supposed to come of itself, and "epidemic" or contagious croup, which was diphtheria. Now, however, these distinctions are swept away, and boards of health require isolation and quarantine against croup exactly as against any other form of diphtheria.

Cases of fatal croup still occasionally occur which cannot be directly traced to other cases of diphtheria, but the vast majority of them are clearly traceable to infection, usually from some case in another child, which was so mild that it was not recognized as diphtheria until the baby became "croupy" and search was made through the family throats for the bacilli.

For years we were in doubt as to the cause of diphtheria. Half a dozen different theories were advanced, bad sewerage, foul air, overcrowding; but it was not until shortly after the Columbus-like discovery, by Robert Koch, of the new continent of bacteriology, that the germ which caused it was arrested, tried, and found guilty, and our real knowledge of and control over the disease began. This was in 1883, when the bacteriologist Klebs discovered the organism, followed a few months later (in 1884) by Loffler, who made valuable additions to our knowledge of it; so that it has ever since been known as the Klebs-Loffler bacillus. This put us upon solid ground, and our progress was both sure and rapid: in ten years our knowledge of the causation, the method of spread, the mode of a.s.sault upon the body-fortress, and last, but not least, the cure, stood out clear cut as a die, a model and a prophecy of what may be hoped for in most other contagious diseases.

Great as is the credit to which bacteriologists are ent.i.tled for this splendid piece of scientific progress, there was another co-laborer, a silent partner, with them in all this triumph, an unsung hero and martyr of science who deserves his meed of praise--the tiny guinea-pig. He well deserves his niche in the temple of fame; and as other races and ages have worshiped the elephant, the snake, and the sacred cow, so this age should erect its temples to the guinea-pig. From one of the most trifling and unimportant,--kept merely as a pet and curiosity by the small boys of all ages,--he has become, after the horse, the cow, the pig, and the sheep, easily our most useful and important domestic animal. It may be urged that he deserves no credit, since his sacrifice--though of inestimable value--was entirely involuntary on his own part; but this should only make us the more deeply bound to acknowledge our obligation to him.

By a stern necessity of fate, which no one regrets more keenly than the laboratory workers themselves, the guinea-pig has had to be used as a stepping-stone for every inch of this progress. Upon it were conducted every one of the experiments whose results widened our knowledge, until we found that this bacillus and no other would cause diphtheria; that instead of getting, like many other disease-germs, into the blood, it chiefly limited itself to growing and multiplying upon a comparatively small patch of the body-surface, most commonly of the throat; that most of its serious and fatal results upon the body were produced, not by the entrance of the germs themselves into the blood, but by the absorption of the toxins or poisons produced by them on the moist surface of the throat, just as the yeast plant will produce alcohol in grape juice or sweet cider.

Here was a most important clew. It was not necessary to fight the germs themselves in every part of the body, but merely to introduce some ferment or chemical substance which would have the power of neutralizing their poison. Instantly attention was turned in this direction, and it was quickly found that if a guinea-pig were injected with a very small dose of the diphtheria toxin and allowed to recover, he would then be able to throw off a still larger dose, until finally, after a number of weeks, he could be given a dose which would have promptly killed him in the beginning of the experiments, but which he now readily resisted and recovered from. Evidently some substance was produced in his blood which was a natural antidote for the toxin, and a little further search quickly resulted in discovering and filtering out of his body the now famous ant.i.toxin. A dose of this injected into another guinea-pig suffering from diphtheria would promptly save its life.

Could this ant.i.toxin be obtained in sufficient amounts to protect the body of a human being? The guinea-pig was so tiny and the process of ant.i.toxin-forming so slow, that we naturally turned to larger animals as a possible source, and here it was quickly found that not only would the goat and the horse develop this antidote substance very quickly and in large amounts, but that a certain amount of it, or a substance acting as an ant.i.toxin, was present in their blood to begin with. Of the two, the horse was found to give both the stronger ant.i.toxin and the larger amounts of it, so that he is now exclusively used for its production.

After his resisting power had been raised to the highest possible pitch by successive injections of increasing doses of the toxin, and his serum (the watery part of the blood which contains the healing body) had been used hundreds and hundreds of times to save the lives of diphtheria-stricken guinea-pigs, and had been shown over and over again to be not merely magically curative but absolutely harmless, it was tried with fear and trembling upon a gasping, struggling, suffocating child, as a last possible resort to save a life otherwise hopelessly doomed. Who could tell whether the "heal-serum," as the Germans call it, would act in a human being as it had upon all the other animals? In agonies of suspense, vibrating between hope and dread, doctors and parents hung over the couch. What was their delight, within a few hours, to see the muscles of the little one begin to relax, the fatal blueness of its lips to diminish, and its breathing become easier. In a few hours more the color had returned to the ashen face and it was breathing quietly. Then it began to cough and to bring up pieces of the loosened membrane that had been strangling it. Another dose was eagerly injected, and within twenty-four hours the child was sleeping peacefully--out of danger. And the most priceless and marvelous life-saving weapon of the century had been placed in the hand of the physician.

Of course there were many disappointments and failures in the earlier cases. Our first ant.i.toxins were too weak and too variable. We were afraid to use them in sufficient doses. Often their injection would not be consented to until the case had become hopeless. But courage and industry have conquered these difficulties one after another, until now the fact that the prompt and intelligent use of ant.i.toxin will effect a cure of from ninety to ninety-five per cent of all cases of diphtheria is as thoroughly established as any other fact in medicine. The ma.s.s of figures from all parts of the world in support of its value has become so overwhelming that it is neither possible nor necessary to specify them in detail. The series of Bayeaux, covering two hundred and thirty thousand cases of diphtheria, chiefly from hospitals and hence of the severest type, showing that the death-rate had been reduced from over _fifty-five_ per cent to below _sixteen_ per cent already, and that this decrease was still continuing, will serve as a fair sample.

Three-quarters of even this sixteen per cent mortality is due to delay in the administration of the ant.i.toxin, as is vividly shown in thousands of cases now on record, cla.s.sified according to the day of the disease on which the ant.i.toxin was given, of which MacCombie's "Report of the London Asylums Board" is a fair type. Of one hundred and eighty-seven cases treated the first day of the disease, none died; of eleven hundred and eighty-six injected on the second day of the disease, four and a half per cent died; of twelve hundred and thirty-three not treated until the third day of the disease, eleven per cent died; of nine hundred and sixty-three cases escaping treatment until the fourth day, seventeen per cent died; while of twelve hundred and sixty not seen until the fifth day, twenty per cent died. In other words, the chances for cure by the ant.i.toxin are in precise proportion to the earliness with which it is administered, and are over four times as great during the first two days of the disease as they are after the fourth day. One "stick" in time saves five.

This brings us sharply to the fact that the most important factor in the cure of diphtheria, just as in the case of tuberculosis, is early recognition. How can this be secured? Here again the bacteriologist comes to our relief, and we needed his aid badly. The symptoms of a mild case of diphtheria for the first two, or even three, days are very much like those of an ordinary sore throat. As a rule, even the well-known membrane does not appear in sufficient amounts to be recognizable by the naked eye until the middle of the second, or sometimes even of the third, day. By any ordinary means, then, of diagnosis, we would often be in doubt as to whether a case were diphtheria or not, until it was both well advanced and had had time to infect other members of the family.

With the help of the laboratory, however, we have a prompt, positive, and simple method of deciding at the very earliest stage. We merely take a sterilized swab of cotton on the end of a wire, rub it gently over the surface of the throat and tonsils, restore it to its gla.s.s tube, smearing it over the surface of some solidified blood-serum placed at the bottom of the tube, close the tube and send it to the nearest laboratory. The culture is put into an incubator at body heat, the germs sown upon the surface of the blood-serum grow and multiply, and in twelve hours a positive diagnosis can be made by examining this growth with a microscope. Often, just smearing the mucus swabbed out of the throat over the surface of a gla.s.s slide, staining this smear, and putting it under a microscope, will enable us to decide within an hour.

These tubes are now provided by all progressive city boards of health, and can be had free of charge at depots scattered all over the city, for use in any doubtful case, within half an hour. Twelve hours later a free report can be had from the public laboratory. If every case of suspicious sore throat in a child were promptly swabbed out, and a smear from the swab examined at a laboratory, it would not be long before diphtheria would be practically exterminated, as smallpox has been by vaccination, and this is what we are working toward and looking forward to.

Our knowledge of the precise cause of diphtheria, the Klebs-Loffler bacillus, has furnished us not only with the cure, but also with the means of preventing its spread. While under certain circ.u.mstances, particularly the presence of moisture and the absence of light, this germ may live and remain virulent for weeks outside of the body, careful study of its behavior under all sorts of conditions has revealed the consoling fact that its vitality outside of the human or some other living animal body is low; so that it is relatively seldom carried from one case to another by articles of clothing, books, or toys, and comparatively seldom even through a third party, except where the latter has come into very close contact with the disease, like a doctor, a nurse, or a mother, or--without disrespect to the preceding--a pet cat or dog.

More than this, the bacillus must chiefly be transmitted in the moist condition and does not float in the air at all, clinging only to such objects as may have become smeared with the mucus from the child's throat, as by being coughed or sneezed upon. As with most of our germ-enemies, sunlight is its deadliest foe, and it will not live more than two or three days exposed to sunshine. So the princ.i.p.al danger against which we must be on our guard is that of direct personal contact, as in kissing, in the use of spoons or cups in common, in the interchange of candy or pencils, or through having the hands or clothing sprayed by a cough or a sneeze.

The bacillus comparatively seldom even gets on the floor or walls of a room where reasonable precautions against coughing and spitting have been taken; but it is, of course, advisable thoroughly to disinfect and sterilize the room of a patient and all its contents with corrosive sublimate and formalin, as a number of cases are on record in which the disease has been carried through books and articles of clothing which had been kept in damp, dark places for several months. The chief method of spread is through unrecognized mild cases of the disease, especially of the nasal form. For this reason boards of health now always insist upon smears being made from the throats and noses of every other child in the family or house where a case of diphtheria is recognized. No small percentages of these are found to be suffering from a mild form of the disease, so slight as to cause them little inconvenience and no interference with their attending school. Unfortunately, a case caught from one of these mild forms may develop into the severest laryngeal type. If a child is running freely at the nose, keep it at home or keep your own child away from it. A profuse nasal discharge is generally infectious, in the case of influenza or other "colds," if not of diphtheria.

This also emphasizes the necessity for a thorough and expert medical inspection of school-children, to prevent these mild cases from spreading disease and death to their fellows. By an intelligent combination of the two methods, home examination of every infected family and strict school inspection, there is little difficulty in stamping out promptly a beginning infection before it has had time to reach the proportions of an epidemic.

One other step makes a.s.surance doubly sure, and that is the prompt injection of all other children and young adults living in the family, where there is a case of diphtheria, with small doses of the ant.i.toxin for preventive purposes. Its value in this respect has been only secondary to its use as a cure. There are now thousands of cases on record of children who had been exposed to diphtheria or were in hospitals where they were in danger of becoming exposed to it, with the delightful result that only a very small per cent of those so protected developed the disease, and of these not a single one died! This protective vaccination, however, cannot be used on a large scale, as in the case of smallpox, for the reason that the period of protection is a comparatively short one, probably not exceeding two or three weeks.