The part played in history by epidemic disease is now so obvious to us that it is strange that anyone, let alone the greatest scholar of his age (and one of the greatest of any age), Edward Gibbon, should have overlooked it.
In the famous 15th chapter of his The History of the Decline and Fall of the Roman Empire, the sceptical and ironical Gibbon asks what accounts for the rise of Christianity:
To this inquiry an obvious but unsatisfactory answer may be returned; that it was owing to the convincing evidence of the doctrine itself. But as truth and reason seldom find so favourable a reception in the world, and as the wisdom of Providence frequently condescends to use the passions of the human heart, and the general circumstances of mankind, as instruments to execute its purpose, we may still be permitted, though with becoming submission, to ask, not indeed what were the first, but what were the secondary causes of the rapid growth of the Christian church?
Gibbon goes on to enumerate the five causes, as he sees them. They are the inflexible zeal of the Christians, the doctrine of future life, the miraculous powers ascribed to the primitive church, the pure and austere morals of the Christians, and the union and discipline of the Christian republic.
Gibbon mentions disease only once, in connection with the doctrine of future life. He says:
A regular series was prepared of all the moral and physical evils which can afflict a flourishing nation; intestine discord, and the invasion of the fiercest barbarians; pestilence and famine, comets and eclipses, earthquakes and inundations.
And that is all: pestilence is put on a level with the appearance of comets that provoke superstitious fear.
The population of Rome, however, declined from one million to 60 000 between ad 150 and 530, largely as a result of epidemic disease. In the century and a half after ad 250 the population of the empire as a whole declined by a half, for the same reason.
In a brilliant book entitled Plagues, Priests and Demons (Cambridge University Press, 2005), Daniel Reff draws a parallel between the evangelisation of the old and new worlds. The demographic collapse in Europe at the time of the adoption of Christianity was mirrored by the demographic collapse in Mesoamerica and Peru with the coming of the Spanish and the diseases that they brought with them. In the latter case, of course, the Spanish did not die from the epidemics of diseases to which they were already immune, but which reduced the population of Mesoamerica by 90% in a hundred years: the Indians can scarcely fail to have noticed this and to have pondered its meaning.
According to Professor Reff, the old beliefs in pagan gods had no explanation for the catastrophic epidemics, no ways of explaining them or of giving them meaning. Moreover, the church offered a new family to those whose own families had been decimated by disease. Thus epidemics were the most powerful evangelisers of all, a factor that the wonderfully perceptive Gibbon (no doubt used to the very high mortality rates of his own society) all but overlooked.
The psychological origination of a belief does not determine its truth or falsehood, of course. Moreover, a believer could perfectly well accept Professor Reff’s thesis by attributing the epidemics themselves to the mysterious working of Providence. The sceptic, of course, would have none of it. No evidence will decide the issue for either of them either way.