# Popular Science Monthly/Volume 4/March 1874/Alternations in the Intensity of Diseases

Layout 4
* ALTERNATIONS IN THE INTENSITY OF DISEASES.
FROM THE FRENCH OF ALPHONSE DE CANDOLLE.
TRANSLATED BY H. H. W.
THE diminution of the efficacy of vaccination, as a preservative from the small-pox, has been the subject, at first of incredulity, and afterward of surprise, to the medical world, and even to the non-professional public. The causes of this change have been sought in the nature of the vaccine matter. But it has not been demonstrated that taking the matter anew from the cow is to restore the primitive efficacy of the remedy.
Without wishing to call in question with the profession the chances of discovering an explanation, drawn from the domain of medical and physiological facts which they occupy, I desire to point out a consequence of the fundamental law of heredity, as applied to the phenomenon in question. In order to understand the subject in its true ​aspects, it will be well, in the first place, to recall a fact in relation to epidemics.
Medical history proves, on the subject of epidemic and contagious maladies, a marked fatality at the time of their first appearance, followed by slowly-decreasing violence from generation to generation. In our own memory the epidemic visits of cholera have diminished in frequency and intensity within a short period of time. Previously to our day, syphilis and varioloid, two infective diseases, differing in their nature, and in their modes of transmission, had presented the same phenomenon—Extreme intensity at the beginning, diminution from period to period.
If this diminution belonged to the nature of the maladies, populations infected for the first time in the nineteenth century should have suffered less than those infected in previous centuries. But this is not what has occurred. When a savage population has recently been visited, for the first time, by the infection of small-pox, it has suffered as much as the Europeans at the beginning of the malady in Europe. It is the fact of invading a new field which renders epidemics destructive. Upon a little reflection, the reason of this is easy to comprehend.
When an epidemic falls upon a population for the first time, the. greater part of the individuals disposed to receive the disease are attacked. They die in great numbers. Subsequent births are the offspring of persons who did not contract the disease, or, at the least, who contracted, yet survived it; that is to say, of persons better constituted than others to resist the disease. By virtue of the ordinary resemblance of children to their parents, the new generation will be less disposed to suffer from the epidemic. There will be then a diminution of the violence of the disease, or a temporary disappearance. For the most part I presume a diminution, because that the resemblance of children to their grandparents (which is called atavism) is not very rare, and tends to reproduce certain forms or physiological conditions in families. At the end of two or three generations, that special cause for the return of the epidemic is less felt, the resemblance to a great-grandfather, or ancestor still more removed, being more rare than the resemblance to a grandfather. But then the bulk of the population will no more have been exposed, by itself, or by its fathers and mothers, to the malady in question, or will have been but slightly exposed. Thus is constituted anew, by the very purity of the disease, a proportion of individuals who have not been submitted to the proof of the infection, or of whom the parents have not been submitted to the test; a proportion on whom the malady will be severe, and among whom the law of selection will recommence to operate.
The law of events (force des choses) introduces then a variation in the intensity of every disease, except that it does not act upon diseases of which people rarely die, or which fall principally upon the ​aged. The more fatal a disease among youth, the quicker is the work of the law of selection, and the more prompt the diminution of the malady. If a first invasion, for instance, destroys a moiety of the population below marriageable age, the survivors should be very little liable, in their physical or physiological conditions, to the disease, and the children born to them will profit by their immunity. If the disease is less fatal, the purification will be less. We thus discover, I do not say the cause, but a cause why pestilences and other very serious maladies attack populations at intervals, and are, as it is said, epidemic; while certain diseases less serious, even among maladies which attack youth, rule from year to year in a mode more continuous.
Such are the clear laws—one might add the rigid laws—which rule in diseases, to produce aggravation or diminution, independently of all these natural circumstances. Without doubt there may be other circumstances, physical or physiological, and physicians may discover preventive or curative means which exert influence upon them. But the incessant effect of heredity, and of the law of selection, exists, notwithstanding; and, when other influences cannot be demonstrated, we may be assured that heredity and selection perform their part.
We now see that the efficacy of preventive means, such as vaccination, should also vary. When Jenner discovered the utility of vaccination, the small-pox had in a slight degree lost, in Europe, its primitive intensity. The people who then existed proceeded from many generations which could, thanks to the process of selection, passably resist the epidemic. Individuals were not so readily affected as at the origin of the disease, or, if they had the disease, they succumbed to it in a smaller proportion; or, yet again, those who survived rarely contracted the disease a second time. It was supposed that those who had the disease by inoculation were sheltered from a repetition, and the dangerous practice of inoculation would not have continued, but for this opinion. Vaccination, then, came at an epoch when the European population found itself in ameliorated conditions with regard to epidemic small-pox. Practised with ardor, it had the effect to render small-pox very rare. But, precisely because it had become rare in the generation which immediately succeeded Jenner, in the generation which issued from that was found a majority composed of persons who had not been exposed to the epidemic. Among them must have been some persons who naturally, or by atavism, were disposed to take the infection. From that cause arose a certain renewed sensitiveness (recrudescence), which vaccination could less easily control.
In other words, after two or three vaccinated generations, the European population having been slightly exposed to the small-pox, found itself approximating to the conditions of a population in which the disease appears for the first time. The attack is not altogether so ​violent, but the return is evident. All means of resisting it, which would have sufficed fifty years since, have become less efficacious.
To sum all in general terms: heredity and selection must produce an alternation of intensity and relief in diseases. That variation must be more marked, when the disease in which it takes place is more fatal, and especially when it attacks youth. Curative or preventive means, which are sufficient in periods of light visitation, lose a portion of their efficacy at the aggravated periods. And this rule applies particularly to the use of vaccine as a preventive of small-pox.
The works of Darwin being now familiar to physicians, it is probable that many among them have considered the effect of the law of selection upon the variation of intensity in maladies. I doubt, how-ever, whether they have given attention to the consequences relative to vaccination. It is this which has led me to bring within the range of medical investigation an application (perhaps novel) of the ideas of the celebrated English naturalist.