By these tables [wrote Dr. Jurin] it appears that upwards of 7 per cent., or somewhat more than a fourteenth part of mankind, die of the smallpox; and consequently the hazard of dying of that distemper, to every individual born into the world, is at least that of 1 in 14.[33]
This large induction from London to universal mankind is noteworthy, because, as we shall see, it came to be often made, and involved a serious fallacy; for unless universal mankind dwelt in conditions similar to Londoners, it was idle to infer a common rate of disease and mortality. The population of London in 1701 was estimated at about 500,000 (there was no exact census), rising to about 600,000 in 1720. It was closely packed and lodged over cess-pools; the water supply was insufficient, and there was no effective drainage. The vast multitude was disposed, as if by design, for the generation and propagation of zymotic disease, and specially smallpox. Little attention was paid to personal cleanliness, and still less to ventilation, to light, to exercise. The condition of a large urban community a century ago is almost inconceivable at the present day. Londoners were then only slowly and blindly rising out of those modes of existence which made the Plague of 1665, and other plagues, possible. Hence we need not be astonished that smallpox was a common and persistent affliction; but it was less prevalent and less deadly than it is the custom to assert; and had the disease not been attended with injury to feminine beauty, there might have been no more fuss made about it than about any other form of eruptive fever.
It has also to be observed, that smallpox as a cause of death was probably much exaggerated in the Bills of Mortality; for as Isaac Massey pointed out--
These Bills are founded on the ignorance or skill of old women, who are the searchers in every parish, and their reports (very often what they are bid to say) must necessarily be very erroneous. Many distempers which prove mortal, are mistaken for the smallpox, namely, scarlet and malignant fevers with eruptions, swinepox, measles, St. Anthony’s fire, and such like appearances, which if they destroy in three or four days (as frequently happeneth) the distemper can only be guessed at, yet is generally put down by the searchers as smallpox, especially if they are told the deceased never had them.[34]