| Period | Sum of all deaths | Per cent. under Two | Per cent. from Two to Five | Per cent. from Five to Ten | Per cent. of Smallpox | Per cent. of Measles | Per cent. of Whooping-cough | Per cent. of “Bowel-hive” | ||||||||
| 1783-88 | 9994 | 39·40 | 10·66 | 3·42 | 19·55 | 0·93 | 4·51 | 6·72 | ||||||||
| 1789-94 | 11103 | 42·38 | 11·90 | 3·79 | 18·22 | 1·17 | 5·13 | 6·43 | ||||||||
| 1795-1800 | 9991 | 38·82 | 12·21 | 3·45 | 18·70 | 2·10 | 5·36 | 6·47 | ||||||||
| 1801-06 | 10304 | 33·50 | 13·43 | 5·10 | 8·90 | 3·92 | 6·12 | 7·27 | ||||||||
| 1807-12 | 13354 | 35·89 | 14·22 | 5·58 | 3·90 | 10·76 | 5·57 | 9·26 |
The actual deaths from smallpox, measles and whooping-cough are shown in the next table, which includes for comparison the corresponding figures from the London bills of mortality:
Smallpox, Measles and Whooping-cough in London and Glasgow, 1783-1812.
| London | Glasgow | |||||||||||
| Year | Smallpox | Measles | Whooping-cough | Smallpox | Measles | Whooping-cough | ||||||
| 1783 | 1550 | 185 | 268 | 155 | 66 | 153 | ||||||
| 1784 | 1759 | 29 | 457 | 425 | 1 | 41 | ||||||
| 1785 | 1999 | 20 | 194 | 218 | 0 | 34 | ||||||
| 1786 | 1210 | 793 | 200 | 348 | 2 | 173 | ||||||
| 1787 | 2418 | 84 | 228 | 410 | 23 | 57 | ||||||
| 1788 | 1101 | 55 | 298 | 399 | 1 | 17 | ||||||
| 1789 | 2077 | 534 | 374 | 366 | 23 | 45 | ||||||
| 1790 | 1617 | 119 | 391 | 336 | 33 | 177 | ||||||
| 1791 | 1747 | 156 | 279 | 607 | 4 | 117 | ||||||
| 1792 | 1568 | 450 | 311 | 202 | 58 | 68 | ||||||
| 1793 | 2382 | 248 | 352 | 389 | 5 | 112 | ||||||
| 1794 | 1913 | 172 | 469 | 235 | 7 | 51 | ||||||
| 1795 | 1040 | 328 | 311 | 402 | 46 | 180 | ||||||
| 1796 | 3548 | 307 | 536 | 177 | 92 | 60 | ||||||
| 1797 | 522 | 222 | 567 | 354 | 5 | 76 | ||||||
| 1798 | 2237 | 196 | 418 | 309 | 3 | 98 | ||||||
| 1799 | 1111 | 223 | 451 | 370 | 43 | 95 | ||||||
| 1800 | 2409 | 395 | 380 | 257 | 21 | 27 | ||||||
| 1801 | 1461 | 136 | 428 | 245 | 8 | 125 | ||||||
| 1802 | 1579 | 559 | 1004 | 156 | 168 | 90 | ||||||
| 1803 | 1202 | 438 | 586 | 194 | 45 | 60 | ||||||
| 1804 | 622 | 619 | 697 | 213 | 27 | 52 | ||||||
| 1805 | 1685 | 523 | 703 | 56 | 90 | 129 | ||||||
| 1806 | 1158 | 530 | 623 | 28 | 56 | 162 | ||||||
| 1807 | 1297 | 452 | 439 | 97 | 16 | 85 | ||||||
| 1808 | 1169 | 1386 | 326 | 51 | 787 | 92 | ||||||
| 1809 | 1163 | 106 | 591 | 159 | 44 | 259 | ||||||
| 1810 | 1198 | 1031 | 449 | 28 | 19 | 147 | ||||||
| 1811 | 751 | 235 | 486 | 109 | 267 | 62 | ||||||
| 1812 | 1287 | 427 | 508 | 78 | 304 | 103 | ||||||
The ratio of deaths under the age of two had decreased greatly in Glasgow, while the ratios from two to five and from five to ten had increased. At the same time smallpox had almost ceased (but only temporarily, as it appeared) to be the great infectious scourge of infancy, while measles had come in its place. “Now that the smallpox are in great measure expelled,” (Watt believed that cowpox inoculation had done this), “the measles are gradually coming to occupy the same ground which they formerly occupied. I am sorry to make this statement, but the facts, at least with regard to Glasgow, are too strong to admit of doubt.”
In order to explain the enormous increase of deaths by measles, he had recourse to the following argument. Formerly nearly all children, say nine-tenths, had both smallpox and measles, the attack of smallpox in most cases coming first. Children who had survived smallpox were fortified by that ordeal, not merely as selected lives, but positively fortified, so that measles, when it assailed them in due time afterwards, was taken mildly or was “modified,” not one in a hundred cases proving fatal. But now (1813), when so few children have been through the smallpox, measles has become ten times more fatal to them, although it could hardly be more common than it used to be. Having found it necessary to assume that children in former times took smallpox before they took measles, nine-tenths of them taking both, he qualifies this in another passage: “Still, however, as the measles came round now and then, as a very general epidemic, they must occasionally have had the precedence, and it was perhaps chiefly among such patients that the disease proved fatal.”
The measles which came round now and then as a general epidemic was nearly the whole of it; even in London there were intervals of several years with only a few annual deaths, and in smaller towns or country districts the clear intervals were longer. The prevalence of measles on the great scale being more casual than that of smallpox, it is likely that most children had taken smallpox before they incurred measles. But it is clear from such instances as the London epidemic of 1674, and the epidemic in the Foundling Hospital in 1763, that measles might attack children just before smallpox, and by its weakening effects, increase the number of victims of the latter. As to the fatality of measles itself in the 17th and 18th centuries, the statement of Watt that it did not amount to one death in a hundred attacks, while it can neither be proved nor disproved by an array of figures, can be shown to be inconsistent with the language of annalists. The epidemics of measles varied in severity then as afterwards: that of 1670 in London was regular and mild, that of 1674 in the very same months of the year was anomalous and fatal; Huxham characterizes the measles at Plymouth in the winter of 1749-50 as “maximé epidemici, imo et saepe pestiferi”; at Kidderminster, in 1756, after fevers had been very fatal to adults, the measles went through the town so that an immense number of children “died tabid”; in the West of England about 1760 a disease called measles made “a melancholy carnage amongst children.”
While Watt’s theory of the working of this principle of substitution is open to criticism on some points of detail, the law itself, as enunciated by him, remains to the present time one of the soundest and most instructive generalities in epidemiology. He based it upon a laborious search of the burial registers, such as no one before him in this country had undertaken. Next he saw correctly that a great rise in the deaths of infants by such a disease as measles could only be accounted for by a great increase in the rate of fatality. Thirdly, he connected the loss from measles with the saving from smallpox. Adopting an old opinion, which may be discovered in Willis[1219], he argued that smallpox, when taken first, served to fortify children so that they passed easily through the measles afterwards; but in the following passage he indicated a better reason why the absence of smallpox gave measles the chance of proving more fatal: “In this point of view we are not to consider the smallpox as so peculiarly fatal in their nature. They perhaps prove so fatal merely by having the start of other diseases. The measles, the chincough, the croup, the scarlet fever, and perhaps many others, would have proved equally fatal had they occurred first.” The principle is true to this extent, that a certain proportion of weakly infants, or children of poor stamina, will succumb to almost any disease—if not to smallpox, then to measles, and if not to measles directly, then to the sequelae of measles. This was perceived in the form of a necessary truth by Haygarth in 1793: “A considerable number of those who now die of the smallpox would die in childhood of other diseases if this distemper were exterminated[1220].” It was commonly believed that smallpox had at length found its real artificial check, not in the inoculation of itself, but in the inoculation of cowpox. At all events it had declined greatly in Glasgow. During the three years before the measles epidemic of 1808, there could hardly have been more than a thousand children attacked by smallpox, or not one in ten of all the children born. During several years the infancy of the city had been spared any great ordeal of infectious disease; the first epidemic that came along happened to be measles, so that it fell to that infection to take off the weaklings. In the economy of nature it is impossible to rear all the young of a species, nor would it be good for the species if it were possible. It is among the birds that the principle of population, or of the survival of the fittest, is seen working in the most admirable way: the annual migration of many species to breed in a remote country brings with it an ordeal for the birds of the year in finding their way to the winter feeding-grounds—an ordeal which only the strongest come through. For some unexplained reason, the young of the human species are peculiarly tried by infectious diseases, which multitudes pass through safely, while many of poor stamina or of ill tending are cut off.
Dr Watt’s teaching, as to the displacement of one infectious cause of death by another was resisted at the time as being of “evil tendency” for the pretensions of vaccination, although Watt believed as firmly in the virtues of cowpox as Jenner himself did. Writing to James Moore on 6 Dec. 1813, Jenner says of Watt’s essay (Baron, II. 392): “There is nothing in its title that developes its purport or evil tendency.... Is not this very shocking? Here is a new and unexpected twig shot forth for the sinking anti-vaccinist to cling to.” Sir Gilbert Blane, who was then president of the Medical and Chirurgical Society, having a natural fondness for ideas of all kinds expressed in a paper to that society rather more approval of Watt’s view than was thought prudent: “An ingenious friend of mine has remarked to me in conversation that some light is thrown on this subject by considering that whichever of the epidemic maladies attack children first, it will be the most fatal, inasmuch as all feeble constitutions will fall in its way while the stronger will be left to encounter the attacks of the others; and that the smallpox, owing probably to the greater abundance and rankness of their effluvia, are generally caught in a casual way before measles, hooping cough and scarlet fever, and are therefore reckoned more fatal than any of these. But, a new field of research being opened,” etc. Efforts were made to correct the effect of this, by showing that measles in some parts of the country had not been more fatal than usual. Holland, of Knutsford, attributed the fatality of the epidemic in 1808 to a change of the wind to the east. Writers in the Edinburgh Med. and Surg. Journal, pointed out that Watt had compared the absolute deaths by smallpox at one time and by measles at another without taking account of the increase of population, and the rates of mortality from each disease. The best criticism of Watt was by Roberton in his Mortality of Children, 1827, p. 49. He offers the following considerations, without seeming to know that they were really to be found in Watt’s own essay: Smallpox used to be caught first; it swept off the feeble and sickly, leaving the strong and vigorous only to encounter the attacks of other diseases. “That infectious febrile disease to which in early infancy there is the strongest predisposition will of course in general make the first attack and prove the most fatal of any.” There were reasons why measles used to have comparatively few victims, “and why, when they now prevail epidemically, they, as was the case with smallpox, are caught at an earlier age than other diseases in general and prove so very fatal: which happens not more from their priority in attack than from being in their nature what they were ever considered—a severe and dangerous disease. We are to recollect, however, that measles do not in general attack at so early an age as smallpox; nor ever, like the latter, destroy eight or nine-tenths of all the children that die in the place where they happen to prevail, as was the case in the variolous epidemics of Chester and Warrington [this is an error, vide supra, p. 554]; consequently we have reason to hope that neither measles nor any other infantile disease will, as Dr Watt imagined, ‘come to occupy the place which smallpox once occupied,’” (p. 58). A feeble echo of Roberton’s criticism, with all its scientific candour left out and its points against Watt emphasized in a spirit of paltry cavilling, was heard next year in the Goulstonian Lectures of Bisset Hawkins on Elements of Medical Statistics, 1829.