The interdependence of the forms of zymotic disease, so luminously displayed by Dr. Watt in the statistics of Glasgow, was apparently a conception in excess of the capacity of the average medical mind. It was the custom of vaccinators to treat smallpox as a solitary existence, any diminution of which was ascribed to the observance of their rite, and any increase to its neglect, although the observance of the rite was neither less in the waxing, nor more in the waning of the disease. With many illustrations from medical literature, Mr. Gibbs enforced the lesson that diseases were not irregular and detached disasters, but varied manifestations of a common disorder: that when one form prevailed, other forms abated or disappeared; that health, and the defect of health, were referable to habits and conditions of life; and that the consequent rate of mortality was unaffected whether smallpox happened to be one of its factors—indeed, in numerous instances, a reduced rate of mortality signalised the prevalence of smallpox. In short, to suppose that the creation and culture of an ailment like vaccination could by any means tend to the invigoration of life was to reverse the canon—that health always and everywhere was the best defence of health.
Among the supporters of his thesis, none had greater weight than Dr. George Gregory. Adducing “the experience which twenty years of official connection with the Smallpox and Vaccination Hospital had given him,” he asserted—
The great principle that there are no diseases strictly isolated from others. They are links in a chain—
“All are but parts of one stupendous whole.”
They must be viewed in conjunction, if we would hope to form just, enlarged, and legitimate views of the character and pathological affinities of each.
Long surmised, but never proved, until the statistical inquiries of recent times showed its correctness, Dr. Gregory continues—
We may, for want of a better name, call this curious doctrine the Law of Vicarious Mortality; by which is understood that whenever one epidemic diminishes, another increases, so that the sum total of epidemic mortality remains, on an average of years, nearly the same.
Epidemic Mortality in England and Wales during 1838, 1839, and 1840.
| Year 1838. | Year 1839. | Year 1840. | |
| Smallpox | 16,268 | 9,131 | 10,434 |
| Measles | 6,514 | 10,937 | 9,326 |
| Scarlet Fever | 5,802 | 10,325 | 19,816 |
| Total Mortality by the Exanthemata, | 28,584 | 30,393 | 39,576 |
| Hooping Cough | 9,107 | 8,165 | 6,132 |
| Total Epidemic Mortality | 37,691 | 38,558 | 45,708 |
| Total Mortality throughout England and Wales | 342,529 | 338,979 | 359,561 |
We learn from this table, that every year is distinguished by some master epidemic. In 1838, Smallpox was the ruling epidemic throughout England. In 1839, Measles and Scarlet Fever struggled for the mastery. In 1840, Scarlet Fever was so general and so fatal, that the mortality by it exceeded by one-fifth the ravages of Smallpox during the epidemic season of 1838, and more than doubled the mortality by that disease in 1839....